The Production Characteristics of Bromus inermis Leyss and Their Inheritance

ADVANCES IN AGRONOMY. VOL. 33

THE PRODUCTION CHARACTERISTICS OF Bromus inermis LEYSS AND THEIR INHERITANCE P. D. Walton Department of Plant Science, The University of Alberta, Edmonton, Alberta, Canada

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1. A. Plant Characters

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........ ................................................ Seed Production and Establishment ...................... ........ A. Floral Initiation . . . . . .................................. B. Stomata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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C. Germination and Seedling Growth . . . . . . . IV.

B. Digestibility . . . ........................................ C. Protein Content ................................. . . . . . . . . . . . . 352 V. Forage Yield . . . . . . . . . . . Vl. A. Seed Yield

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B. Dry Matter Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . .

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I. INTRODUCTION A. PLANTCHARACTERS

The genus Bromus comprises about 60 species, both introduced and native to North America. The name Bromus, which is the Greek for oat, is indicative of 341

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the nature of the panicle and the area of adaptation. Bromus inermis Leyss, which is known as bromegrass, Austrian brome, Hungarian brome, Russian brome, and most commonly, smooth bromegrass, belongs to the subgenus Zerna of the tribe Festuceae. It is normally an octoploid (2n = 8 X = 56), and two distinct types are recognized within the species. The southern type was introduced from France and Hungary into the United States and was later brought to Canada, where it is grown in the eastern part of the country and in southern British Columbia. The northern type was introduced to Canada directly from Germany in 1888 and is now extensively grown in the prairie provinces. Smooth bromegrass is a native of Europe and Asia and is adapted to most temperate climates. It is a leafy, sod-forming perennial which spreads vegetatively by underground rhizomes. It is resistant to drought and to extremes of temperature, being capable of withstanding both hot, dry summers and long, cold winters. The species is grown both alone and in mixtures with other grasses and legumes and is used for pasture, hay, and erosion control. The forage quality of smooth bromegrass ranks well among the cool-season grasses. The crude protein content is high, ranging from 12 to over 20%, during the time of rapid growth at the beginning of the season. The inflorescence consists of many spikelets, each of which contains several hermaphroditic florets. The pollen is disseminated by wind. Each panicle usually produces abundant seed and is highly crossfertilizing, naturally cross-pollinated, and rather self-sterile.

B. AGRICULTURAL USE Following the introduction of smooth bromegrass into the North American continent and the initial recognition of the value of this species, interest declined. It was not until farmers and scientists sought to combat the dust-bowl conditions generated in the early 1930s by a combination of extensive ploughing, overgrazing, and drought conditions that it was realized that smooth bromegrass was, among the introduced grasses, one of the principal and most widespread survivors. It was as a result of selection work and studies carried out in those years that the two types of bromegrass (northern and southern) were recognized and described. The species is now accepted as one of the most successful grasses used for erosion control on roadside shoulders and steep road cuts. It is frequently grown in mixtures with other grasses for the vegetation of waterways, irrigation canal banks, and terraces, as well as in areas where the soil has been extensively disturbed. The aggressive and extensive root system of this grass species brings about rapid improvement in soil structure. Where a legume has been incorporated, the decay of the legume root maintains a balance of available nitrogen and aids in the decomposition of the grass roots. In areas where subsoil has been exposed or where soil has been eroded on slopes, the establishment of the species is much enhanced by a dressing of a nitrogenous fertilizer.

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On the Canadian prairies, smooth bromegrass is one of the principal grass components of a grass-alfalfa seed mixture which is widely used on nonirrigated pastures. Frequently, such fields are first harvested for hay, whereas the regrowth is used for pasture. The most favorable time for the establishment of smooth bromegrass is in the early spring. Planting should be carried out as early as the establishment of arable crops will permit. Under Canadian farming conditions, both a companion crop or excessive weed growth will retard the establishment of such a spring sowing. Competition for nutrients or moisture may be reduced by early and repeated mowings at such a height that the bromegrass plants will not be clipped. Material cut in that way should be removed so as not to cover the seedlings. Where pastures are used for grazing only, rotational grazing will double the production of beef per hectare over that obtained from continuous grazing (Walton, 1979). Where continuous grazing is necessary smooth bromegrass stands should be understocked during the early part of the season to allow growth to accumulate for later use. Such practices do, however, present dangers, since forage quality decreases as the plants mature. Like many other tall grasses, the yield of smooth bromegrass decreases with frequent cutting (Wright er al., 1967). On the Canadian prairies, four defoliations, under grazing conditions, and two cuts, where the material is intended for hay, will give optimum productivity and a satisfactory balance between production and forage quality. For both hay and pasture, smooth bromegrass behaves satisfactorily, if not ideally, in a mixture with alfalfa. Bromegrass makes a substantial contribution to early harvests. Where haying is delayed so that the first harvest is taken during or after the time of bromegrass seed formation, alfalfa will increase and dominate the mixture (Walton, 1979). Where grazing is frequent or stocking rates are high, alfalfa will tend to be grazed out of the mixture. Maintaining a balance between the grass and the legume in the pasture is important, since the danger of bloat increases as the proportion of alfalfa becomes higher. However, the legume will improve palatability and intake of the feed, resulting in better animal performance.

II. THE NATURE OF THE SPECIES A. PLOIDYLEVELSA N D CHROMOSOME NUMBERS

Smooth bromegrass is most often encountered as an octoploid (2n = 8x = 56), although a range of chromosome numbers (2n = 28, 42, 56, and 70 and some intermediate values), suggesting a polyploid series, has been reported. It appears that an active state of evolution still exists in what may well be an old polyploid complex. The perennial nature of the species, together with its persis-

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tent and aggressive growth habit, has perpetuated a number of chromosome aberrations of the type reported by La Flew and Jalal(l972) and Tan and Dunn ( 1977a). Initial attempts to determine the genomic constitution of the octoploid led to disagreement, and three main theories emerged. Hill and Carnahan (1957) considered it to be an autoalloploid, with the genome formula AAAA BBBB, in which A and B represent two distinct genomes. The high bivalent and low quadrivalent frequencies in the plants examined were attributed to a genetic mechanism suppressing quadrivalent formation between homologous chromosomes. Ghosh and Knowles (1964), Schulz-Schaeffer (1960), and Wilton (1965) observed that the chromosome complement contained three pairs of satellite chromosomes, whereas either two or four pairs might have been expected from the above theory. Two pairs of chromosomes, of similar appearance, bore large satellites, whereas the third pair bore minute satellites. On this basis the genome formula of AAAABBCC was proposed. Elliott and Love (1948), Elliott (1949), and Nielsen (1951) suggested an alloploid genome formula (A, A,A,A,B, B, B, B 2) in which homologous genomes had been differentiated by gross structural morphology. The low frequency of quadrivalent formation indicated a remnant homology of parental genomes. By means of interspecific hybrids between B . erectus (2n = 28) and B . inermis, Armstrong (1977) demonstrated that the genome of B . erectus, arbitrarily designated as the A genome, was a component in the tetrasomic condition of B. inermis. This genome consists of five median chromosomes, one subterminal chromosome, and one chromosome with a large satellite. In interspecific hybrids between B. arvensis (2n = 14) and B . inermis, the haploid genome of B . inermis could be studied, since the chromosomes of B. arvensis are much larger than those of B . inermis. The karyotype of the B genome, constructed in this manner, consisted of two pairs of subterminal and five pairs of median chromosomes. From the results of this study, Armstrong concluded that there was no evidence to indicate the presence of more than two genomes. In explaining the opposing conclusions reached by previous workers, Armstrong points out that polymorphic chromosome morphology is not unknown in cross-pollinated species; also, suppression of at least one pair of satellites, the possible differences in contraction of chromosomes, and mechanical stresses during preparation of the slides may influence the interpretation of the genome. However, he concedes that the conventional staining procedures which he used would not be expected to detect homologous genome differentiation. The Giemsa staining technique could, through the banding patterns it produces, detect karyotype differences between homologous genomes. Within the genus, recent studies have shown that it is possible to cross Bromus inermis with B . compelianus (Hanna, 1961), B . erectus (Armstrong, 1973), and B . arvensis (Armstrong, 1977). In all cases the hybrids were infertile.

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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 ( 4 x , 6 x , and 8 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.

2 . 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.

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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

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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

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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.

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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 6 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 2 to 8 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-

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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 do 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

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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).

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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 4 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

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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

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these studies do 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).

2 . 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

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(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:

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1. High application of fertilizer or high soil fertility 2. Drought conditions 3. 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

+

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(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. 3 . 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. 4 . 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 ( 1 968) 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.

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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.

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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: 1. Tiller size and weight 2. Plant height and yield of the second cut 3. Leaf area 4. Winter survival and first harvest yield 5. Plant height early in the growing season 6. 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

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bromegrass forage yield and determined a plant ideotype for maximum forage yield from this species, when harvested twice a year. For the first and second harvest, “a dense population of nonelongated tillers with a high leaf weight and area, early in the season,” followed by a rapid increase in the number of longstemmed headed tillers would give the most promising yields. The same authors (Walton and Murchison, 1979b) also showed that for all growth stages, tiller weight has positively correlated with leaf area and standard leaf weight. Nonelongated tiller density increased when nonelongated tiller leaf numbers per tiller were high. The density of elongated and headed tillers increased when stems were long, whereas high standard leaf weight, stem weight, and leaf weight reduced tiller density. Tan ef al. (1976a) also showed that tiller density and tiller dry weight were two major components of forage yield. Many research workers, Ross et af. (1970), Mishra and Drolsom (1972a,b), Walton (1974a,b, 1976), Tan ef al. (1976a,b), Timothy et al. (1959), Robinson and Thomas (1963), and Dunn and Wright (1970) have found that most of the morphological traits associated with forage yield in smooth bromegrass give general combining ability mean square values larger than those for specific combining ability. Knowles (1950), Drolsom and Nielsen (1970), Mishra and Drolsom (1972b), Sleper and Drolsom ( 1974), and Tan et al. (1 977) were able to show that for some traits, specific combining ability was also important. The proportion of phenotypic variation that is heritable is measured by calculating the broad-sense heritability (H ,,), while narrow-sense heritability (H ”) measures the proportion of the total genetic variation due to additive genetic variance. The following values have been obtained for total annual yield: Hb

0.68 0.48

HIl 0.25 0.37

Reference Tan er al. (1977) Walton and Murchison (1979~)

Heritability values for a range of plant characters are shown in Table I. The narrow-sense and broad-sense heritability values (Table I) indicate that, while the values for yield itself are low, plant breeders should be able to make substantial progress in changing plant height, leaf length, leaf width, panicle length, stomatal length and frequency, tiller density, sheath vein number, sheath width, and leaf digestibility. Walton (1974a) has drawn attention to discrepancies in size of the additive genetic variance as determined by Griffin’s (1956) analysis and that shown by narrow-sense heritability estimates. Both methods would be influences (in different ways) by the meiotic irregularities reported for this octoploid species, while high ploidy levels themselves will also affect the calculation of these values. Heritability estimates depend on the type of material, experimental design, and the reproductive system involved. However, since these factors influence all characters equally, the relative ranking of the traits is of interest.

36 1

CHARACTERISTICS OF Bromus inermis LEYSS

Table I Heritability Character Leaf characters Leaf width Vein number Vein frequency Interveinal distance Stomatal length Stomatal frequency Leaf number Leaf length Leaf area Leaf angle Leaf rigidity Tiller density Canopy height Sheath characters Sheath width Vein number Vein frequency Interveinal distance Stomatal length Stomatal frequency Panicle characters Culm diameter x lo2 Panicle length x 10 Spikelets/panicle Florets/spikelet x lo2 Panicles/plant x 10 First harvest Forage yield Leaf-blade dry weightkiller Leaf-sheath dry weight tiller Stem dry weighthiller Acid detergent fiber Whole plant Leaf blade Leaf sheath Stem Crude protein Second harvest Forage yield Acid detergent fiber (whole plant) Crude protein Plant height

Hb

H"

0.80 0.84 0.49

0.62 0.68

0.46

;:"0

0.72 0.73 0.75 0.74 0.94 0.48 0.59 0.86 0.88

0.38 0.55 0.14

0.80 0.71 0.48 0.48 0.72 0.59

0.21 0.33 0.30

0.45 0.94 0.59 0.42 0.48

0.27 0.48

0.68 0.94 0.95 0.92

0.25 0.57 0.56 0.46

0.77 0.93 0.85 0.83 0.58

0.04

0.49 0.31 0.16 0.54

Reference

Tan et al. (1976a)

0.49

0.44

Tan er al. (1977)

Tan et al. (1976a)

Mishra and Drolsom (1972b)

0.46 0.19 0

\

Tan et al. (1978a)

0.03 0.22 0.15 0.08 0.44

Walton and Murchison (1979~)

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P. D. WALTON

2 . Canopy Characteristics The canopy architecture of a forage crop influences efficient light utilization, photosynthetic activity and, consequently, crop productivity. Tan el a f . ( 1977) used pathway coefficient analysis to demonstrate that the two canopy characters that showed the closest positive association with the spring forage yield in smooth bromegrass were the leaf area per tiller and tiller density. There was a negative direct effect between yield and other characters such as leaf number, leaf length, leaf angle and rigidity, and canopy height. Leaf area per tiller and tiller density exerted opposite influences in most of the association pairs measured by path coefficient analysis. Those working with other forage crop species have also been confronted with this dilemma. Rhodes (1972), following a suggestion by Cooper and Edwards (1961), successfully used selection for high critical leaf area index as the means of overcoming the problem. The relationship between leaf area per tiller and tiller density reached its own equilibrium under such a selection program. Teare (1972), who used a formula that included leaf area index, leaf angle, and canopy height to measure the relationship between these characters and the attenuation of radiant energy in smooth bromegrass, found that the best single component to predict light attenuation was forage area per unit of soil surface (i.e., leaf area index). His data supported the hypothesis that a tall, erect growth habit (with leaves distributed evenly along toe culm) is conducive to uniform illumination of leaf area within the forage canopy and this, in turn, would lead to high yields. 3 . Genotype Interaction with the Environment

Perennial forage grasses are expected to yield well under climatic and management conditions that are much more diverse than those provided for any other crop. These broad expectations make the nature of genotype by environment interaction particularly important. Since this interaction is generally accepted as being large, it is surprising that few studies have been undertaken to quantify, for the grasses, the magnitude of their genotypic interaction with environment. An exception exists in the case of smooth bromegrass. Tan et al. studied the effect of genotype by environment interaction on forage yield (1979a) and on morphological characters associated with yield (1979b). These trials were carried out at four locations in the province of Alberta, Canada, using a seven-parent diallel without reciprocals. For most of the characters studied, specific and general combining ability gave highly significant interactions with environments. Evidently, there was a differential expression of gene action at the four locations. Tan et al. (1979~)further partitioned genotype by environment interaction into heterogeneity among regressions and the residual, thus enabling environments to

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363

be assessed in two ways: first, as a mean expression of all genotypes and second, as the average performance of several parental genotypes. The first method showed that, for all the characters studied, a substantial part of the interaction was derived from heterogeneity among regression lines. However, since the residual components were frequently significant, some unpredictable variation was present. The linear model would, however, be predictive of the behavior of characters such as tiller density, yield per area, and second harvest yield, since tests of heterogeneity for the regression lines were highly significant. Considering next the second approach, when environments were assessed using the parental means, it was found that the levels of significance differed considerably from those determined by the first method. However, the ranking of genotypes on the basis of their linear regression coefficients was similar, so that the way in which the environment was measured did not affect the conclusions that might be drawn from the regression data. For all characters, a significant part of the genotype by environment interaction was due to the heterogeneity of the regression lines. However, the predictability of the genotypes varied with the characters measured. It is evident that the various forms of regression analysis provide powerful tools which can elucidate complex genotype by environment interactions and transform them into a series of predictable linear responses. The most sophisticated of these methods, the Eberhardt and Russell (1966) modification of the Finlay and Wilkinson (1963) model, generates three parameters: (1) the regression coefficient, (2) the deviation from the mean square value, and (3) cultivar performance. The problem of determining the weighting to be attached to each of these statistics has not been solved.

IV. PLANT BREEDING Many of the studies and publications discussed in the previous sections were conducted and written to present basic information from which breeding programs could be developed. Consequently, the consideration of possible plant breeding programs for smooth bromegrass will summarize and draw conclusions from the material already presented. The forage breeder is interested in obtaining agronomic improvement in three areas: (1) seed yield, (2) dry matter yield, and (3) forage quality. These three topics will be considered in that order. A. SEEDYIELD

Knowles and Ghosh (1968) studied the isolation distances necessary to prevent interpollination in Bromus inermis by using a genetic marker; they showed that

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P. D.WALTON

for isolation distances of 1, 61, and 183 meters, the average contamination of their plots was 9.6, 1.0, and 0.2%, respectively. Thus, the isolation distances required for seed multiplication under the Canada Seed Act are marginal to maintain cultivar purity. However, the extent of contamination could be reduced considerably if borders were removed and discarded prior to harvest. The possibility of using plant breeding techniques to improve both seed yield and seed quality in smooth bromegrass was considered by Knowles et al. (1970). Under the environmental conditions prevailing in Saskatoon where they worked, Knowles and associates were able to show that for southern bromegrass strains, there was a marked response to selection for higher seed yields. In the selected progenies, aneuploid plants (2n = 55) had been eliminated, accounting for much of the seed yield improvement in the selection lines over the base population. Progenies of crosses between northern and southern bromegrass types also yielded promising material when selected for improved seed set. Almost invariably, seed set was higher in the progeny of southern bromegrass types that had been outcrossed with northern cultivars. In no case did a decline in forage yield accompany improved seed set. Heritability for seed yield and seed quality were high, and the authors believed that continued mass selection would prove successful in improving these characters. The evidence discussed in Section II1,C indicates that selection for seedling vigor in smooth bromegrass could well be achieved by screening breeding material for high seed weight. Considered from the viewpoint of long-term research, plant breeding methods provide the most promising approach to increased seed production. For the seed producer, however, an adequate level of soil fertility, wide row spacings, and protection from insects and diseases should prove most satisfactory in achieving high seed production. B . DRYMATTERYIELD

Mass or recurrent selection for dry matter yield is a simple approach that has been used successfully in many grass breeding programs. When practiced in Edmonton, Alberta, it was found that the repeatability of yield results from year to year was poor; clones that gave high yields in one year ranked poorly in the next. Evidently, a more sophisticated approach is required. The poor repeatability may be explained partially by the low narrow-sense heritability values (about 0.25) obtained for forage yield, when expressed as dry matter (Tan et al., 1977, 1978a,b,c; Walton, 1974a; Walton and Murchison, 1979c; see Table I). Also, both additive and nonadditive genetic variances have been shown to be of importance for first, second, and total forage harvest yield (Walton, 1974a; Tan et al., 1977, 1978a,b,c; Walton and Murchison, 1979~).Further, in some cases (Tan et al., 1979a), these two types of genetic variance have been shown to be of equal importance. In addition, Tan et al. (1979a) showed that the interaction between

CHARACTERISTICS OF Bromus

inermis LEYSS

365

the environment and both general and specific combining ability was highly significant. All these influences would impede direct selection for yield. Such difficulties could be overcome by using a recurrent selection program combined with testing at a number of locations. Another way of obtaining consistently high-yielding strains by direct selection would be to divide the area for which the new cultivars were intended into ecological zones and develop a plant breeding program for each region, but this would be costly. Instead of making direct selections for forage yield, the use of yield-related characters or yield components shows promise. This view is supported by the many publications (listed in Section V,B) that show general combining ability is high for morphological traits and by the results of the pathway analysis conducted by Tan et al. (1979b). The selection for leaf area, tiller weight, and tiller density may well give more substantial yield increases than direct selection for yield. However, selection for large leaf area, high dry weight, values per tiller, and high tiller density may pose some difficulties, since tiller density was negatively associated with leaf area (-0.46) and tiller dry weight (-0.61). Large populations should be studied to determine if these opposing characters are separable. If a choice had to be made, using existing information, the emphasis should be placed on tiller density (Tan et al., 1976a; Walton, 1976; Walton and Murchison, 1979a,b,c). Yield increases might also be achieved by simultaneous selection for canopy characters, which have been shown by Tan et al. (1977) to be closely correlated with yield. Such characters may also be manipulated by management practices and other environmental influences. Also, in some cases, the narrow-sense heritability values are low for canopy characters (leaf angle = 0.06). The character most closely associated with forage yield is plant height (Walton, 1974a,b; Tan et al., 1976a,b); however, this character is also much influenced by environment (Tan et al., 1979b) and, hence, unsuitable for selection purposes. Of the plant characters studied, those that provide the most satisfactory alternative for selection purposes to direct forage yield, are the stomata and vein characters discussed in Section II,B,2. For both the stomata (Walton, 1974b) and vein (Tan et al., 1976a) characters studied, the nonadditive genetic variances were substantial. Under these circumstances, hybrid cultivars (two-clone synthetics) should provide a satisfactory expression of the desired traits. Hybrid material of this type could be simply adapted to a range of ecological zones of the type suggested earlier. C. FORAGEQUALITY

It is only in very recent years that plant breeders have given attention to forage quality. A six-clone diallel cross was used to investigate the inheritance of in vitro digestibility by Ross et al. (1970). A diallel cross analysis of data collected

366

P. D.WALTON

in two seasons showed that additive genetic effects were present and that general combining ability was a significant source of variation. Sleper et al. (1973) used the acid-pepsin dry matter disappearance technique to study the heritability of forage digestibility. General combining ability was highly significant, again indicating that additive gene action was more important than nonadditive gene action in the inheritance of this trait. These authors agreed with Bhat and Christie (1975) in concluding that significant progress could be made by selecting for high digestibilities within bromegrass populations. None of these findings, however, was in agreement with the results obtained by Kamstra et al. (1973), who was able to show that two cloned synthetics, which had been selected for high and low in v i m and in vivo digestibilities, showed no significant differences for those characters. Also, Christie (1977) found that while the correlation between parents and progeny for in vitro digestibility values was high (0.55),one cycle of phenotypic selection was ineffective in improving digestibility. Most recently, Tan and associates (1978a) confirmed earlier findings which showed that general combining ability was a significant source of the genetic variation for digestibility in bromegrass. These authors also found that the acid detergent fiber and the crude protein contents of whole plants showed nonadditive genetic effects, while leaf blade acid detergent fiber was the only character that gave a high narrow-sense heritability. With that exception, narrow-sense heritability values for quality characteristics were all less than 10%.Under these circumstances, advances due to genetic selection would be slow. It is not surprising that while the genetic evidence indicates that selection for improved digestibility in smooth bromegrass should be possible, such attempts as have been reported in the literature have not been successful. A number of authors attempted to overcome this difficulty by using morphological traits, rather than chemical analysis, as criteria for selection (Bhat and Christie, 1975; Sleper and Drolsom, 1974; and Christie and Mowat, 1968). Digestibility, stem diameter, and leaf width have been reported to be positively correlated, while negative correlations have been found between plant height and digestibility. Tan er al. (1976a) believed that wide interveinal distances could increase digestibility. There are, however, no reports of the approach being used in a plant breeding program. Obviously, the improvement of nutritional value by plant breeding methods introduces fundamental problems that still remain to be resolved. Possibly the most serious of these is the lack of an accurate and rapid method of determining forage quality. Shenk (1977) has drawn attention to the value of using a computerized system of spectroscopy, pointing out that this method has the capability of analyzing large numbers of samples for multiple quality factors. Such a system would be capable of simultaneously determining crude protein, acid detergent fiber, lignin, cellulose, and nonstructural carbohydrates. While such techniques may be of considerable value, the plant breeder is still faced with the problem of relating the

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quality characteristics that he has studied to yield and yield-related traits, which also form a part of his breeding program. In the case of long-lived perennial plants which must be evaluated over a number of seasons and which are markedly influenced by environmental factors in both time and space, this can be a formidable task. It is, however, a problem that both plant and animal breeders have considered for many years. In view of the low heritabilities encountered in forage quality characteristics, it is advisable that this be taken into account in weighing the different traits for which selection is to be made. This is, in fact, the process of developing a selection index for a series of traits weighted for differences in their heritabilities. Computer models, which have been developed for this purpose (Shenk, 1975), could be used on data from progeny tests of large populations of parental clones evaluated for forage yield and quality over a range of locations, harvests, and generations.

REFERENCES Akin, D. E., and Burdick, D. 1975. Crop. Sci. 15, 661-668. Armstrong, K. C. 1973. Can. J . Genet. Cytol. 15, 427-436. Armstrong, K. C. 1977. Z. Pjlanzenzuecht. 79, 6-13. Baker, B. S., and Jung, G. A. 1968. Agron. J. 60, 155-158. Barta, A. L. 1975. Crop. Sci. 15, 169-171. Bhat, A. N., and Christie, B. R. 1975. Crop. Sci. 15, 676-679. Calder, F. W. 1977. Can. J . Plant Sci. 57, 441-449. Canode, C. L. 1968. Agron. J . 60, 263-267. Canode, C. L., Anwar Maun, M., and Teare, I. D. 1972. Crop Sci. 12, 19-22. Christie, B. R. 1977. Can. J . Plant Sci. 57, 57-68. Christie, B. R., and Mowat, D. N. 1968. Can. J . Plant Sci. 48, 67-73. Clarke, J. M., and Elliott, C. R. 1974. Can. J . Plant Sci. 54, 475-477. Cooper, J. P., and Edwards, K. J. R. 1961. Heredity 16, 63-82. Drolsorn, P. N., and Nielsen, E. L. 1969. Crop Sci. 9, 710-713. Drolsorn, P. N., and Neilsen, E. L. 1970. Crop Sci. 10, 17-18. Dunn, G . M., and Wright, J. A. 1970. CropSci. 10, 56-58. Eberhart, S. A,, and Russell, W. A. 1966. Crop Sci. 6, 36-40. Elliott, F. C. 1949. Agron. J . 41, 298-303. Elliott, F. C., and Love, R. M. 1948. Agron. J . 40, 335-341. Finlay, K. W., and Wilkinson, G. N. 1963. Aust. J. Agric. Res. 14, 742-754. Follett, R. F., Power, J. F., Grunes. D. L., Hewes, D. A., and Mayland, H. F. 1975. Agron. J . 67, 819-824.

Fuehring. H. D., Mazaheri, A,, Bybordi, M., and Khan, A. K. S. 1966. Agron. J . 58, 195-198. Fulkerson, R. S. 1972. Can. J . Plant Sci. 52, 613-618. Genest, J., and Steppler, H. 1973. Can. J . Plant Sci. 53, 285-290. Ghosh, A. N.. and Knowles, R. P. 1964. Can. J . Genet. Cyrol. 6, 221-231. Griffin, B. 1956. Aust. J. Biol. Sci. 9, 463-493. Gross, C. F., and Jung, G. A. 1978. Agron. J . 70, 397-403. Gross, D. F., Mankin, C. J., and Ross, J. G. 1975. Crop Sci. 15. 273-275. Hanna, R. M. 1961. Can. J. Botany 39, 757-773.

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