ADVANCES IN AGRONOMY VOLUME 33 CONTRIBUTORS TO THIS VOLUME R W ARNOLD F H BEINROTH R J BURESH F B CADY M E CASSELMAN MARLING CLINE G D FARQUHAR R J HAYNES ERNESTA KIRKBY T M MCCALLA KONRADMENGEL W H PATRICK,JR J A SILVA H T STALKER G UEHARA P W UNGER P D WALTON R WETSELAAR ADVANCES IN AGRONOMY Prepared in cooperation with the AMERICAN SOCIETY OF AGRONOMY VOLUME 33 Edited by N C BRADY International Rice Research Institute Manila, Philippines ADVISORY BOARD H J GORZ,CHAIRMAN R.B GROSSMAN T M STARLING I B POWELL J W BIGGAR M A TABATABAI M STELLY, EX OFFICIO, ASA Headquarters 1980 ACADEMIC PRESS A Subsidiary of Harcourr Brace Jovanovich, Publishers New York London Toronto Sydney San Francisco COPYRIGHT @ 1980, BY ACADEMIC PRESS, INC ALL RIGHTS RESERVED NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER ACADEMIC PRESS,INC I I Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC ( L O N D O N ) LTD 24/ 28 Oval Road, London NWI 7DX LIBRARY OF CONGRESS CATALOG CARD NUMBER:50-5598 ISBN 0-12-000733-9 PRINTED IN THE UNITED STATES OF AMERICA 80 81 82 83 CONTENTS CONTIUBUTORS TO VOLUME 33 PREFACE ix xi CONSERVATION TILLAGE SYSTEMS P W Unger and T M McCalla I Introduction I1 Historical III Tillage Equipment and Use IV Crop Yields and Quality V Environmental Consideration VI Infiltration and Water Conservation VII Weed Control VIII Insects and Plant Diseases IX Soil Temperature X Soil Structure and Other Physical Properties XI Chemical Effects and Microbial Activity XI1 Economics XI11 Summary and Conclusions References 13 16 30 35 38 39 43 48 49 51 53 POTASSIUM IN CROP PRODUCTION Konrad Mengel and Ernest A Kirkby I I1 111 IV V Introduction Potassium Availability in the Soil Potassium in Physiology Potassium Application and Crop Growth Conclusions References 59 60 74 91 103 103 UTILIZATION OF WILD SPECIES FOR CROP IMPROVEMENT H T Stalker I Introduction I1 Biosystematics V 112 113 vi CONTENTS 111 The Gap between Hybridization and Utilization IV Approaches for Utilizing Wild Germplasm Resources V Examples of Species Used in Wild Species Hybridization Programs VI Specific Uses of Wild Species for Crop Improvement VII Summary and Conclusions References 118 119 126 135 140 141 NITROGEN FIXATION IN FLOODED SOIL SYSTEMS A REVIEW R J Buresh M E Casselman and W H Patrick Jr I Introduction I1 Nitrogen Fixation in the Water Column and on the Soil Surface 111 Nitrogen Fixation in the Aerobic Layer of Flooded Soil IV Nitrogen Fixation in the Anaerobic Layer of Flooded Soil V Nitrogen Fixation in the Root Zone of Nonnodulated Plants VI Nitrogen Fixation on the Leaf and Stem Surface of Plants VII Environmental Factors Influencing Nitrogen Fixation in Flooded Soil VIII Comparison of Acetylene Reduction and 15N Methodology IX * Contribution of Fixed Nitrogen to the Nitrogen Requirements of Plants X Perspectives References 150 152 160 161 163 170 174 180 183 185 187 EXPERIENCE WITH SOIL TAXONOMY OF THE UNITED STATES Marlin G Cline I Introduction I1 General Reactions to the System 111 Use of Soil Taxonomy Internationally IV Problems for Users of Soil Taxonomy V Taxonomic Problems VI Impact of Soil Taxonomy Internationally VII Summary References 193 195 197 202 207 213 222 223 vii CONTENTS COMPETITIVE ASPECTS OF THE GRASS-LEGUME ASSOCIATION R J Haynes I Introduction I1 Competition in the Pasture Community 111 Physiological Considerations IV Morphological Considerations V Competition for Environmental Factors VI Conclusions References 227 229 231 239 248 254 256 NITROGEN LOSSES FROM TOPS OF PLANTS R Wetselaar and G D Farquhar I Introduction 263 264 111 Possible Pathways of Nitrogen Losses from Tops 279 IV Associated Methodology Problems 293 V Conclusions 298 References 299 I1 Record of Observed Decreases of Nitrogen Content of Plant Tops AGROTECHNOLOGY TRANSFER IN THE TROPICS BASED ON SOIL TAXONOMY F H Beinroth G Uehara J A Silva R W Arnold and F B Cady I Introduction 304 304 Soil Classification in Perspective 309 Agrotechnology Transference Research 316 Quantitative Verification of Transferability within a Soil Family 323 Prerequisites for Worldwide Agrotechnology Transfer 332 Conclusion: Implication for Agricultural Development 336 References 338 I1 The Transfer of Agrotechnology 111 IV V VI VII THE PRODUCTION CHARACTERISTICS OF Bromus inerrnis LEYSS AND THEIR INHERITANCE P D Walton I Introduction I1 The Nature of the Species 341 343 viii CONTENTS III Seed Production and Establishment IV Forage Quality V Forage Yield VI Plant Breeding References 347 350 353 363 367 INDEX 371 CONTRIBUTORS Numbers in parentheses indicate the pages on which the authors’ contributions begin R W ARNOLD (303), Department of Agronomy, New York State College of Agriculture and Life Sciences, Cornell University, lthaca, New York 14853 F H BEINROTH (303), Department of Agronomy and Soils, College of Agricultural Sciences, University of Puerto Rico, Mayaguez, Puerto Rico 00708 R J BURESH* (149), Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State University, Baton Rouge, Louisiana 70803 F B CADY (303), Biometrics Unit, Department of Plant Breeding and Biometry, New York State College of Agriculture and Life Sciences, Cornell University, lthaca, New York 14853 M E CASSELMAN (149), Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State University, Baton Rouge, Louisiana 70803 MARLIN G CLINE ( 93), Department of Agronomy, Cornell University, lthaca, New York 14853 G D FARQUHAR (263), Department of Environmental Biology, Research School of Biological Sciences, Australian National University, P.O Box 47.5, Canberra City, Australia 2601 R J HAYNES (227), Department of Soil Science, Lincoln College, Canterbury, New Zealand ERNEST A KIRKBY (59), Department of Plant Sciences, The University, Leeds LS2 9JT, England T M MCCALLA ( l ) , Agricultural Research, Science and Education Administration, USDA, University of Nebraska, Lincoln, Nebraska 68583 KONRAD MENGEL (59), Institute of Plant Nutrition, Justus Liebig University, 0-6300 Giessen, Federal Republic of Germany W H PATRICK, JR (149), Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State University, Baton Rouge, Louisiana 70803 J A SILVA (303), Department of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu, Hawaii 96822 H T STALKER ( 1 l), Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27650 *Resent address: International Fertilizer Development Center, P.O Box 2040, Muscle Shoals, Alabama 35660 iX CHARACTERISTICS OF Bromus inermis LEYSS 345 Tan and Dunn (1977b) found a high frequency of anaphase irregularities in tetraploid and hexaploid B inermis individuals The commercial octoploid was more stable than plants at the other two ploidy levels The authors believed that chromosomal instability in somatic cells could account for interplant variations of some morphological characters B STOMATA I Relationships with Ploidy Level Although the size of the pollen grains, stomata, and cell, and the plant height and leaf width and length all increase at the higher ploidy levels, not all of these characters are equally reliable in detecting differences in ploidy level Tan and Dunn (1973) studied pollen grain size and stomata size frequency They found that higher correlation coefficient values and smaller standard deviations were obtained at the three ploidy levels ( x , x , and X ) for stomata length than for any of the other characters Both mature plants grown in the field and seedlings grown in the greenhouse conformed to this pattern By studying the length of the stomata on the cotyledons, these authors were able to screen populations of Bromus inermis for plants with different ploidy levels Relationships with Plant Physiology Walton (1974b) first drew attention to the association between high yield, low stomatal frequency, and large stomatal size Three important plant processes are effected by the nature and activity of the stomata First, the photosynthetic efficiency of the leaf and leaf sheath depends on carbon dioxide uptake and oxygen liberation Second, plant respiration releases, together with carbon dioxide, energy stored in the form of carbohydrates Third, the translocation and distribution of both assimilates and breakdown products is influenced by the movement of water vapor When light is not limiting, as is normally the case on the prairies, these fundamental plant processes determine herbage production The synthesis and breakdown of storage products and the translocation of these subtances for both storage and utilization is vital to plant growth rate, development, and, in perennial plants, to winter survival Extensive consideration (Fuehring et al., 1966; Waggoner, 1969) has been given to the function of the stomata in relation to crop water use Tan and Dunn (1975) confirmed conclusions reached by Walton (1974b) and showed that, though stomatal size was relatively constant over most of the plant’s surface, stomatal frequency tended to vary at different locations The same point on equivalent leaves should, consequently, be used to record stomatal frequency 346 P D WALTON Tan and Dunn (1975) were also able to demonstrate that large stomata, long, wide leaves, and high tiller weight were associated with stomatal frequency In 1976, the same authors found that these characters were under genetic control In all cases, specific combining ability was much smaller than general combining ability The narrow-sense heritabilities for both stomatal length (0.69) and stomatal frequency (0.85) indicated that these characters were highly heritable The authors interpreted their data as indicating that selection for low stomatal number and larger stomatal size would produce progeny with a high tiller weight and long, wide leaves Tan et al (1976a) studied the genetics of leaf vein characters and the association of these characters with stomata traits Their evidence indicated that forage yield increases might be obtained at both the first and second harvest by selecting for increased numbers of vascular bundles per unit area of leaf or sheath width (i.e., narrow interveinal distances) Where interveinal distances are small, the proximity of mesophyll cells to vascular tissue should lead to more efficient translocation, enhanced photosynthetic capacity, and increased crop productivity Lea et al (1977a) also studied stomatal diffusion resistance at three ploidy levels These workers were able to show that stomatal resistance was negatively correlated with total stomatal aperture Since porometric measurements of stomatal resistance were more readily obtained than microscopic measurements of stornatal size, the authors advocated the use of porometric measurements as a means of examining the large populations essential to a plant breeding program They pointed out that such measurements would provide an insight into stomatal activity as well as stomatal aperture size Lea et al (1977b) used a “stomatal index” (the percentage ratio of the stomatal frequency to the total number of epidermal cells and stomata on the plant’s surface) to study stomatal activity The advantage of using a stornatal index was that it was unaffected by environmental conditions, whereas stomatal frequency was influenced by the environment through its effect on the growth of leaf blades Lines that had larger stomatal size but similar stomatal indices would be expected to have higher diffusion rates C SELF-FERTILITY In themain, plants of smooth bromegrass are cross-pollinated This lack of selffertility is the outcome of two distince causes First, structural differences in the chromosomes resulting from inversions and translocations have caused meiotic irregularities Second, further irregularities arise from physiological imbalance which is frequently the outcome of hybridization Studies of meiotic irregularity, pollen viability, and seed set have been carried out in self- and openpollinated progenies by Jalal and Nielsen (1965) These authors found low seed set to be associated with differences in the chromosome structure, which resulted CHARACTERISTICS OF Bromus inermis LEYSS 347 in unorientated and lagging chromosomes, bridges, or micronuclei Such structural differences in the chromosomes were not significantly correlated with pollen viability Seed set was also uninfluenced by the proportion of stainable pollen Metabolic or physiological causes appeared to explain anomalous behavior such as prophase pycnosis or stickiness at metaphase 11, since these occurrences were independent of structural differences among the chromosomes Such factors are closely related to pollen grain abortion, as well as being associated with each other In some cases, meiotic irregularity is associated with self-fertility Drolsom and Nielsen (1969) believed that such irregularity also conferred the opportunity for gene interchange and would result in the production of new and possibly desirable combinations They also showed that the progeny of self-pollinated plants were uniform Only a small portion of the potential genetic combinations was expressed Studies carried out by Nielsen and Drolsom (1972) showed that the progeny resulting from diallel crosses were also much more uniform than might be expected from crosses of the highly heterogeneous parents of an octoploid such as smooth bromegrass Cytological studies of the early stages of meiosis of intergeneric and interspecific hybrids showed that many sporocytes aborted during the early stages In other plants, nonfunctional pollen grains were shown to undergo successful meiotic divisions The experimental synthetics produced from a number of bromegrass breeding programs have given disappointing forage yields after being passed through three or four generations of seed multiplications This is surprising, since the Hardy-Weinberg law indicates that there should be zygotic equilibrium after a single generation of panmixis and that there should be no decline in vigor following the second synthetic generation Evidently, one of the prerequisites for the Hardy-Weinberg law is not being fulfilled; either mating is not completely at random, or differential selections of zygotes exist While the influence of natural selection cannot be entirely disregarded, evidence provided by Nielsen and Drolsom (19721, Mishra and Drolsom (1973b), and Pattanyak and Drolsom (1974) supports the hypothesis that nonrandom mating may be responsible for reductions in forage yield following seed multiplication and also for the uniform progeny produced from crossing and selfing Ill SEED PRODUCTION AND ESTABLISHMENT A FLORALINITIATION There are few reported studies of floral initiations for smooth bromegrass Much of the information presented in the literature has been derived by inference 348 P D WALTON from work with other cool-season grasses Canode et al (1972) dissected tillers, at weekly intervals, from bromegrass plants grown under field conditions They were able to show that in Pullman, Washington, on February 12, the first signs of floral development occurred Clarke and Elliot (1974) obtained similar results in Northern Alberta, leading them to believe that both northern and southern types of bromegrass undergo floral initiation exclusively in the spring However, Newell (1951), working in Nebraska, was able to show that smooth bromegrass plants did not head when moved from the field in August to long photoperiods in the greenhouse Some heading took place when the plants were moved on September 15, but moving the plants on November 15 or December 15 produced the most heads This indicates that fall induction had been extensive The methods used by Clarke and Elliot (1 974) were such that induction could have taken place in the fall but the morphological manifestation of this was not evident before the following spring However, Sass and Skogman (1951) showed that floral primordia initiated in the autumn did not, under field conditions, survive the winter, so that for all practical purposes, Clarke and Elliott’s conclusions were sound In Northern Alberta, plants of smooth bromegrass are fully headed in midJune, pollinated by early July, and mature in mid-August Under these circumstances, southern ecotypes of smooth bromegrass produce lower seed yields than the northern ecotypes This contrasts with yield data from Nebraska and other parts of the United States, where the reverse is the case Clarke and Elliot (1974) claimed that since there were no differences between the two ecotypes in the time or degree of floral initiation, the seed yield differences were due to environmental adaption This view is supported by Russian workers (Romanova and Vasiliskov, 1974) who found that in extreme northern latitudes, seed set was frequently reduced and some cultivars produced no seed It is evident that precise information concerning the environmental factors that bring about floral initiation in smooth bromegrass, and the time when they function, is lacking As is common in most cool-season grasses, existing evidence indicates a need for short photoperiods and cool temperatures Also, the environmental requirements for induction differ between the various cultivars and terms B SEEDYIELD Working in Washington, Canode (1968) found that a wide row spacing (up to 60 cm apart) gave increased smooth bromegrass seed yields Spacing between 60 and 90 cm produced neither an increase nor a decrease in seed yield Fulkerson (1972), using close row spacings (35 and 71 cm apart), presented evidence to show that low density plant populations resulted in high seed yields The aggressive nature and creeping habit of smooth bromegrass calls for regular interrow cultivation to maintain high seed production CHARACTERISTICS OF Bromus inermis LEYSS 349 In an attempt to determine whether seed multiplication in northern areas modified the performance of southern types of smooth bromegrass, Knowles and Christie (1972) compared, at two Canadian locations, the original breeder or foundation seed from Nebraska, New York, and Iowa, with seed produced in western Canada The authors studied forage yield, height of growth, and seed volume weight, and found that for the Saskatoon site there were no significant differences; such significant differences as were observed in the forage yield of the different seed lots in the Guelph trial were very small The use of multiplication areas in western Canadian provinces would in no way reduce production potential Use of seed produced there could not be regarded as being detrimental to forage production in eastern Canadian provinces C GERMINATION A N D SEEDLING GROWTH After three cycles of recurrent selections for high seed set, Trupp and Carlson (1971) achieved a substantial increase in that character, accompanied by an increase in seedling vigor, plant height, and disease resistance The average realized heritabilities for seed weight were 42% in both the first and second selection cycles The advantage in seedling vigor which large-seeded lines showed over small-seeded lines diminished as growth and development progressed, but large-seeded plants had a slightly higher forage yield in the first year of harvest McElgunn (1974) showed that germination was initiated more rapidly where alternating, rather than constant, temperature (2°C for 12 hours, followed by 13°C for 12 hours) conditions prevailed The resulting growth rate differences did not persist for longer than days after germination started The Russian workers Kirshin and Shitova (1972) compared seeds of Bromus inermis germinated in the dark with those germinated with exposure to incandescent light for periods varying from to hours The incandescent light inhibited coleoptile growth In general, the degree of inhibition was proportional to the length of exposure, but very short periods of illumination had no effect The authors also showed that a close positive correlation exists between seeding depth and coleoptile size, as well as between the size of the first leaf and the coleoptile length Variations in both of these characters could be obtained either by varying the planting depth or by excluding light In general, studies of germination and germination rates with smooth bromegrass indicate a close positive correlation between seed size and seedling vigor, as well as emphasizing the importance of planting depth in forage establishment Tan et al (1978b), using artificial growth conditions, demonstrated a highly significant correlation between seedling characters such as tiller number, leaf area, leaf number, shoot-to-root ratio, and seedling dry weight and the relative growth rate of the plant Since none of these seedling characters were signifi- 350 P D WALTON cantly correlated with the net assimilation rate, which together with the leaf area ratio constitutes the relative growth rate, leaf area ratio and relative growth rate must be regarded as the important contributors to dry matter production Further work (Tan et al., 1978c) demonstrated a significant and positive correlation between relative growth rate and net assimilation rate (0.72 to 0.98) Thus, the relative growth rate is determined by the net assimilation rate rather than the leaf area ratio, an important consideration in breeding for seedling vigor The effect of a companion crop on the germination of undersown forage seedlings was studied by Genest and Steppler (1973) Where forages were established without a companion crop, soil moisture percentages were higher Because, in this experiment, the light intercepted by the weed growth was greater than that intercepted by the companion crop, differences in forage yields observed by these authors reflected the greater advantage of improved soil moisture conditions rather than better light penetration IV FORAGE QUALITY There are few circumstances under which pastures cannot provide a more economical feed for domestic livestock than any other cropping system However, they not always meet all the dietary needs of the grazing animals Consequently, forage quality is a factor of great economic importance The problem in determining forage quality is that the chemical constituents which are of importance to the grazing animal are difficult to analyze and measure accurately The wide range of chemical methods that may be used to study forage quality usually measure some characteristic that is of indirect importance in animal nutrition The methods themselves are frequently far from rapid, and since forage material is exceedingly variable by nature, accurate determinations call for a large number of samples Consequently, if costs are to be reduced by feeding animals from pasture without losses due to animal ill health, there is a need to develop rapid methods for studying forage quality A ANIMAL FEEDING TRIALS In the absence of satisfactory chemical methods of analysis, animal feeding trials, though expensive, are exceedingly important and informative Using dairy heifers, Martin and Donker (1968) compared, over a 4-year period, animals’ preference for, and production from, smooth bromegrass and Reed canary grass swards The heifers’ average daily gain (0.74 kg/ha) was identical after grazing from the two types of pastures In the absence of choice, the animals’ preference CHARACTERISTICS OF Bromus inermis LEYSS 35 for smooth bromegrass was of little practical significance In fact, since Reed canary grass was higher yielding and more persistent at the trial location (Minnesota), it was, for those circumstances, to be recommended over smooth bromegrass The chemical composition and nutritive value of smooth bromegrass was compared with that of timothy and orchard grass (Dactylisglomerutu), when all three were harvested at 50% inflorescence emergence, by Kureger et al (1969), using dairy goats as test animals Orchard grass, followed by smooth bromegrass, gave the highest values for digestibility of dry matter, crude protein, and acid detergent fiber Smooth bromegrass gave the highest crude protein and in vitro dry matter digestibility values and the lowest percentage acid detergent fiber when the plant parts (leaf blade, leaf sheath, and stem) were compared Calder (1 977) made silage from smooth bromegrass pastures harvested at a range of dates in July and August He concluded that the largest economic return, but not the highest yield of dry matter, would be obtained when the material was cut at the vegetative stage The performance of steers fed early-cut silage compared favorably with the results obtained with high energy concentrates Many beef producers harvest crops intended for silage at the early head stage with a view to obtaining a higher yield; Calder’s evidence indicates that earlier cutting would provide better returns B DIGESTIBILITY Smooth bromegrass was one of the species studied by Wurster et al (1971) to determine in virro and in vivo digestibilities for a range of forage materials The in vitro and in vivo dry matter digestibility values were highly correlated (R = 0.89) Comparisons of digestibility values for whole plants and plant parts collected over the growing season showed significant differences between the two smooth bromegrass cultivars Sac and Manchar The authors believed that this association existed Plant breeders should be able to select bromegrass strains with highly digestible stems and leaf sheaths Such cultivars would be of value where it was intended to harvest bromegrass at a time when the dry matter yield was at a maximum (i.e., for hay) On the other hand, selections for high whole plant digestibility values for early growth stages would produce bromegrass strains suitable for pasture Kunelius et ul (1974) harvested pastures over a 3-year period, at eight developmental stages prior to the first harvest, and at two intervals during the regrowth In vitro digestibility percentages declined as the season progressed and were higher when the growth period was shortest This seasonal decline was most marked during the first year Acid-pepsin dry matter disappearance was positively associated with leaf width, culm diameter, and dark green color All were characters that showed a negative correlation with plant height (Sleper and Drolsom, 1974) 352 P D.WALTON In general, plant disease reduced forage quality Gross et al (1975) were able to demonstrate negative correlation (-0.46) between the percentage of the plant that was diseased and the percentage of in vitro digestible dry matter Inoculation with Drechslera bromi (Died.) and Rhyncosporiurn secalis (Oud.) produced a significant decrease in in vitro digestible dry matter, but inoculation with bromegrass mosaic virus produced no change in forage quality Bhat and Christie (1975) harvested a range of bromegrass genotypes at three stages of maturity and subjected the stems to acid detergent fiber, lignin, cellulose, silica, and total cell wall constituent analysis Top and bottom portions of the stems were analyzed separately The authors showed that in vitro digestibility values declined at the rate of about 1% per day between the time of head emergence and full head extension In the period between the beginning of fully extended head stage and anthesis, in vitro digestibility values declined more slowly (0.1 % per day) The lower parts of the plants were to 8% less digestible than the top parts The bottom part of the plant gave higher percentage values for all cell wall components with the exception of silica There was also a significant negative correlation between in vitro digestibility and the amounts of all cell wall components When the authors calculated the phenotypic and genotypic correlations, high positive values were obtained between the percentage lignin and the percentage acid detergent fiber It seems that the lignin content of the plant can be used as a criterion in a breeding program where selections are aimed at an increase in in vitro digestibility Scanning electron microscope studies were used by Akin and Burdick (1975) to investigate relative rates of digestion for the various types of tissues found in the leaf blade of a range of tropical and temperate grasses, including Bromus inermis In all cases, the mesophyll and the phloem degenerated first The leaf laminas of the temperate-season grasses degenerated much more readily than those of the tropical (C-4) plants, since the vascular bundles occupy a higher percentage of the leaf area in the C-4 species, which explains why temperate grasses are more readily digestible, and hence more nutritious, than the tropical grasses C PROTEIN CONTENT Quantitative and qualitative measurements of the main plant components of smooth bromegrass when grown on irrigated land in the Canadian Prairies were determined by Kilcher and Troelsen (1973) Sequential sampling during a 14week period showed that the highest yield of dry matter was obtained at flowering time and that the proportion of leaves (by weight) decreased to 40% when the plant was mature The leaves contained 12% more crude protein than did the stem over most of the life of the plant When the plant was mature, the cell wall CHARACTERISTICS OF Bromus inermis LEYSS 353 lignin content of the stem was about 70%, whereas that of the leaves did not exceed 60% At maturity, the digestibility of the leaves and stems declined, respectively, to 57 and 35% Nutrient energy yields were highest from material harvested for hay during the 2-week period from heading to midbloom These conclusions were supported by work carried out by Lawrence et al (197 1) and Winch et a f (1970) Both studies showed that early harvesting resulted in a substantially higher protein content Differences in forage quality between a number of species, including smooth bromegrass, have been reported by Tingle and Elliott (1975) These authors did not detect cultivar differences within species The use of an orange dye binding method to determine protein content was discussed by Smith and Lutwick (1975) Although it was possible to calculate a regression relationship between total nitrogen content as determined by conventional methods and the values obtained from the orange dye binding method, variations around the regression line were high when nitrogen contents were greater than 2.5% The orange dye binding method is not satisfkctory for the determination of total nitrogen contents of the grasses V FORAGE YIELD A ENVIRONMENTAL INFLUENCES I Harvest Date Two factors are of importance in determining date of harvest First are quality characteristics, discussed in the previous section, which decline as the season advances Second is dry matter production, which increases toward the end of the growth period If the harvest date is early, the productivity and persistence of the pasture may be decreased In the case of smooth bromegrass, this decrease is not as marked as that found in many other forage species; some authors (Horrocks and Washko, 1968) have found that, whereas productivity was reduced by early harvesting, persistence was in no way affected Kunelius et al (1974) studied the effect of cutting management on smooth bromegrass in eastern Canada over a 3-year period Harvesting prior to heading reduced the forage yield and crude protein production These results confirmed earlier findings by Winch et al (1970) and Rochat and Gervais (1975) In western Canada, McElgunn et al (1972) used a smooth bromegrass and alfalfa sward to test the effect of six defoliation schedules in which the initial cutting date varied Over a 5-year period, these authors were able to demonstrate that early defoliation was detrimental to yield The conclusions drawn from 354 P D WALTON these studies not agree with the results of the animal feeding trials (Calder, 1977) presented in Section IV This difference could well indicate that the protein analysis methods used by McElgunn et al were unreliable Further south, in Wisconsin, Smith et al (1973, 1974) showed that frequent and severe cutting regimes were capable of substantially reducing the porportion of bromegrass in an alfalfa-bromegrass mixture Where the pasture was harvested only once or twice during the year, the height of cut did not result in a significant difference in either yield or persistence These findings in Wisconsin were the reverse of those in western Canada, where repeated cutting or overgrazing eliminates alfalfa from a bromegrass-alfalfa sward (Walton, 1979) Fertilizers While forage crops are now universally accepted as being vitally important for the maintenance of man’s livestock, and consequently for his welfare, the economics of hay and pasture production are such that they have frequently been relegated to poor or marginal lands Hence, the possibilities for large yield increases by improving soil fertility and pH levels are enormous The rates of fertilization commonly used for forage crops in the North American continent are grossly inadequate Undoubtedly, one reason for inadequate fertilization is that livestock producers tend to underutilize the additional forage that fertilization could provide Increases in the capital value of land in recent years make it essential that the maximum profit per hectare be obtained by combining the highest possible yields of good quality forage with efficient and full utilization (Morgan, 1971) It is doubtful if the low levels of productivity that are expected from the native rangelands of the Canadian prairie provinces and parts of British Columbia will continue to support economically viable farming systems A great many workers have shown that nitrogen will consistently increase yields of smooth bromegrass haylands and pastures provided that soil moisture is adequate This work has been summarized by Wedin (1974) Offutt and Hileman (1972) pointed out that the ranking of cultivars from both northern and southern bromegrass types remained unchanged by applications of nitrogenous fertilizer Northern ecotypes in Canada showed a smaller response to fertilization than did the southern cultivars As well as adequate soil moisture, high carbohydrate reserve levels during the fall and winter are essential for maximum responses to nitrogen fertilization in the following year When fertility is high in the fall and both potassium and nitrogen are present, carbohydrate reserves increase, but high rates of potassium without nitrogen cause a reduction in reserves In the spring and summer, nitrogenous fertilizer applications result in the utilization of photosynthates for the production of new top growth, and the development of roots and rhizomes for the accumulation of carbohydrate reserves does not take place CHARACTERISTICS OF Bromus inermis LEYSS 355 (Paulsen and Smith, 1969) Thus, nitrogen fertilizers can reduce the “sodbound” condition of northern bromegrass and has been reported to be more effective than cultivation (Meyer et al., 1977) Meyer et al (1977) also studied forage production, percentage crude protein, and nitrate nitrogen responses in smooth bromegrass over a 22-year period in North Dakota They concluded that applications of nitrogen at rates of about 100 kg/ha were essential to obtain economical production of smooth bromegrass Nitrogen applied at the rate of 66 kg/ha resulted in a 214% increase of yield of dry matter over that of nonfertilized bromegrass An application of 133 kg/ha resulted in a yield increase 257% higher than the control The higher rate of nitrogen application would be economical if the additional crude protein that the herbage contained was replacing an expensive protein supplement Productivity was also increased by repeated annual applications of nitrogen, so that for the last seven years, forage yields and crude protein production were higher than at the beginning of the experiment It is recommended that in areas where the previous year’s forage production was low and where rainfall was minimal, nitrogen application should be reduced, since it might be expected that there would be a nutrient carry-over Where the reverse is the case, nitrogen rates should be increased accordingly Thus, adequate nitrogen fertilization will maintain longterm productivity Hanson et al (1978) were able to show that split applications of nitrogen fertilizer (equal parts in the spring and after the first harvest) gave higher yields and led to a larger total nitrogen recovery than did a single application at the beginning of the season In these trials, conducted under irrigated conditions, both the yield and the percentage recovery from smooth bromegrass were higher than that from the other species tested (Reed canary grass, creeping foxtail) Schou and Tesar (1977) compared applications of anhydrous ammonia with ammonium nitrate on a number of different cool-season grasses Whereas both forms of nitrogen resulted in similar yield increases, the anhydrous ammonia acted more slowly Lechtenberg et al (1974) compared beef production from smooth bromegrass pastures fertilized with anhydrous ammonia with that from pastures fertilized with ammonium nitrate They agreed that anhydrous ammonia and ammonium nitrate were effective in increasing animal production per hectare No animal disorders were observed a Nitrate Poisoning Classical symptoms of nitrate poisoning seldom occur until diets contain in excess of 0.35 to 0.45% nitrate-nitrogen, but the animal’s response to nitrate poisoning is influenced by other components of the ration, particularly the availability of carbohydrate The plant produces nitrates because the first step in protein synthesis involves the use of that substance Consequently, anything that influences protein synthesis may well result in the accumulation of nitrates in plant tissues The most common causes are: 356 P D WALTON High application of fertilizer or high soil fertility Drought conditions Damage to plant tissue by defoliation as a result of grazing or hail damage The more frequent use of nitrogen fertilizers in recent years has made it important that the factors which govern nitrate accumulation be well defined Vanderlip and Pesek (1970) have shown that while nitrate accumulation in the plant increased with rates of application of a nitrogenous fertilizer, the amounts accumulated varied among forage crops Rates of up to 100 kg of nitrogen per hectare resulted in no serious nitrate accumulation in smooth bromegrass Similar applications to orchard grass pastures would raise levels of nitrate-nitrogen to 0.74% The amount of nitrate present in the forage varied considerably in relation to the time of harvesting Potassium deficiency has also been shown to cause an accumulation of nitrates The influence of this substance on nitrate accumulation was most variable and depended on the relative levels of both nitrogen and phosphorus As Vanderlip and Pesek (1970) pointed out, all three major plant nutrients (N, P, and K) affect the nitrate content of forage material In their experiments, MacLeod and MacLeod (1974) showed that under conditions prevailing in eastern Canada, high rates of a nitrogenous fertilizer (896 kg/ha) could increase the percentage of nitrate in smooth bromegrass to 0.43% The rate of potash application had no effect on the percentage nitrate present in the herbage Smith and Lutwick (1975) extended these studies to a range of forage species which were compared at three maturity stages and at four rates of nitrogen fertilizer (0 to 940 kg/ha) All six of the grasses tested could, in the stages prior to heading, accumulate dangerous levels of nitrate in the plant tissue if high dressings of fertilizer were used Russian wild ryegrass showed the greatest increase in nitrates in response to fertilizer, whereas timothy accumulated the least Smooth bromegrass was intermediate b Hypomugnesemiu Grass tetany (hypomagnesemia), caused by low levels of magnesium in an animal’s blood serum, occurs when ruminants graze lush spring pasture The disease appears to be associated with both poor absorption of magnesium in the animal’s intestinal tract and low levels of magnesium in the forage ingested Thill and George (1975) showed that smooth bromegrass, Kentucky bluegrass, crested wheatgrass, tall wheatgrass, and meadow foxtail were less likely than other species tested to cause gas tetany in ruminants Under all circumstances, grasses with K+ to (Ca+ Mg+) cation ratios exceeding 2.2 would place animals at a greater risk than forages, such as those listed above, that had lower cation ratios (Gross and Jung, 1978) Thill and George also presented evidence to indicate that the risk of hypomagnesemia in grazing ruminants was greater during periods of temperature fluctuation A high level of long-chain fatty acids in the herbage was an important factor in increasing incidence of gas tetany + CHARACTERISTICS OF Bromus inermis LEYSS 357 (Barta, 1975), since they reduce magnesium availability The mean percentage of long-chain fatty acids in bromegrass was greater (46%) than in orchard grass (25%) Follett et al (1975) studied the chemical composition of bromegrass on the Northern Great Plains in relation to their gas tetany hazard These authors concluded that, whereas the addition of nitrogenous fertilizers increased enormously the forage production potential and consequently the livestock carrying capacity, nitrogen fertilization might result in gas tetany in ruminants Improved management practices, which might include the oral supplementation of magnesium intake for ruminant livestock, were needed if advantage was to be taken of increased forage production following nitrogen fertilization Interaction between Harvest Date and Fertilizers Smooth bromegrass, in common with other cool-season grasses, has a critical growth stage at which carbohydrate reserves are low and tillers are few (June et al., 1974) This coincides with the time when elongation of the apical meristem has just occurred Intense defoliation at this point in the plant's life can easily lead to a reduction of the plant population Paulsen and Smith (1968) showed that smooth bromegrass grown with alfalfa produced higher yields with frequent (five) cuts than with infrequent (three) cuts The response to these management treatments was reversed when bromegrass was grown by itself, indicating the importance of maintaining fertility levels when pastures are cut frequently Applications of a nitrogenous fertilizer increased both the rapidity of regrowth and the total yield Tiller numbers and tiller growth rate both increased so that photosynthetic areas were quickly replaced Thus, while applications of nitrogen will increase the vegetative yield of smooth bromegrass, as was evident from Section V,A,2, frequent cutting can be detrimental to carbohydrate storage and winter survival Soil Temperature and Moisture Compared with Reed canary grass, smooth bromegrass gives higher yields at the beginning of the season, but shows a poorer yield of regrowth toward the end of the season Read and Ashford ( 968) studied the effect of soil temperature on these two species and found differences in response Although the yield of both species was reduced at lower soil temperatures, the decrease was greater in Reed canary grass These reductions took place under high soil fertility and good soil moisture conditions In both cases the reduction in yield appeared to be the outcome of an inability on the part of the roots to take up nitrogen and phosphorus The yield response of the two species to different levels of phosphate fertilizer was similar Baker and Jung (1968) showed that for smooth bromegrass the optimum daytime temperature for top growth was between 18.3" and 24.9"C 358 P D WALTON Temperatures of 34.8"C, for all the species tested (timothy, orchard grass, and Kentucky bluegrass), gave decreased yields This decrease was less for smooth bromegrass than for any other species For all species an increase in night temperature from 18" to 18.3"C decreased the level of carbohydrate reserves Morrow and Power (1979) conducted trials in which the air temperature was held constant and soil temperature was varied from 3.3" to 33.3"C The optimum soil temperature for smooth bromegrass aboveground dry matter production was 18.3"C At this temperature, most of the other grasses (crested wheatgrass, western wheatgrass, Alta wild rye, Russian wild rye, green needlegrass, side oats grama, and blue grama grasses) produced the greatest amount of root dry matter Waddington (1973) used factor analysis in an attempt to determine the effect of meteorological variables on forage growth in the spring and on regrowth after cutting The most important variable was precipitation Snowmelt provided adequate moisture for early spring growth, so that soil nutrients and temperature were the major factors influencing growth at that time In late spring and early summer, growth was mainly dependent on the rainfall, but temperatures were sometimes low Temperatures from mid-June to mid-August were usually satisfactory or high for forage growth, so that during this period, production was entirely dependent on rainfall Yield differences due to climate were much larger than differences between species B PLANTMORPHOLOGY In the past, agronomists have attempted to study yield by dividing it into components The weight of a given number of grains, the grains per head, and the heads per unit area have all been extensively studied in relation to a number of environment and management factors Frequently, it was found that a factor that enhanced one yield component was detrimental to all or some of the others The study of yield components appeared to be an unrewarding way of determining the underlying dependence structure for yield potential As an alternative, agronomists and plant breeders have studied the associations between morphological characters and yield As well as elucidating the dependence structure underlying yield, such studies might indicate genetic causes that arise from pleiotropic gene action or indicate changes brought about by natural selection or by selection in a breeding program A number of statistical techniques, which include simple and multiple correlations (Walton, 1976), (Tan et al., 1976b), stepwise multiple regression analysis (Walton and Murchison, 1979a), path coefficient analysis (Mishra and Drolsom, 1973b; Tan et al., 1977), and factor analysis (Waddington, 1973; Walton, 1974b, 1976), have been used in these studies CHARACTERISTICS OF Eromus inermis LEYSS 359 I Traits Associated with Yield and Their Genetics The single trait that has most frequently been reported to be closely correlated with the forage yield of smooth bromegrass is plant height (Mishra and Drolsom, 1973; Walton and Murchison, 1979a,b; Tan er al., 1976a,b) Mishra and Drolsom (1973) showed that there was a close positive phenotypic correlation between plant height and leaf weight, leaf length, leaf width, culm diameter, panicle length, and the number of spikelets per panicle These same authors (1972b) also showed that there was a strong positive association between vegetative traits and certain reproductive characters This is surprising in view of the well-known negative association between forage production and seed yield Following their pathway analysis study of phenotypic correlations, Mishra and Drolsom (1973) drew attention to the decrease in the number of spikelets per panicle which accompanied an increase in the number of florets They believed that the two characters underwent simultaneous, mutually exclusive development, competing for the utilization of biosynthetic products The reverse relationship was evident in the development of leaf width and culm diameter; plants with wide leaves had thick culms The authors believed that the photosynthates available at the time when these characters were developing would influence the size of both traits In 1976, Walton, using factor analysis, was able to detect six factors that contributed to the total forage yield of smooth bromegrass in any one year Ranked in descending order of importance, these were: Tiller size and weight Plant height and yield of the second cut Leaf area Winter survival and first harvest yield Plant height early in the growing season Leaf-stem ratio This confirms evidence gathered in earlier studies (Walton, 1974a,b) Tan et al (1976a,b) drew attention to the association between net assimilation rate, crop growth rate, leaf area index, and standard leaf weight and forage yield They related these characters to vein number, stomata size, and stomata1 frequency There was evidence to show that plants with a large leaf area, wide leaves, and a high specific leaf weight had higher vein numbers per unit width of leaf and larger, but fewer, stomata This information is important, for although high forage yield depends on the buildup of labile assimilates, the utilization of these substances in turn depends on the speed of their mobilization and translocation to sink areas Walton and Murchison (1979a) used tiller characters to predict smooth ... countries We express sincere appreciation to them for their efforts N C BRADY ADVANCES IN AGRONOMY VOLUME 33 This Page Intentionally Left Blank ADVANCES IN AGRONOMY, VOL 33 CONSERVATION TlLLAGE... R WETSELAAR ADVANCES IN AGRONOMY Prepared in cooperation with the AMERICAN SOCIETY OF AGRONOMY VOLUME 33 Edited by N C BRADY International Rice Research Institute Manila, Philippines ADVISORY... cultivator and other type machines Mixing and inverting disk machines One-way flexible disk harrow, one-way disk, tandem disk, offset disk Inverting machines Moldboard and inclined disk plow a From Anderson