Intergenotypic competition in mixed stands of spring wheat genotypes

Summary Grain yield and yield components were studied in the three spring wheat genotypes HD 2160 (dwarf), Kalyansona (semi-dwarf) and C 306 (tall) in pure stands as well as in their binary mixed stands during two years. The grain yields of the three genotypes in pure stands ranked as follows: HD 2160 > Kalyansona > C 306. Four mixed stands, 3:1 HD:K, 1:1 HD:C in the first year and 1:1 K:C, 1:3 K:C in the second year out-yielded the pure stand of the better component genotype by 4.4, 2.7, 3.3 and 0.8 percent, respectively. Out of the nine mixed stands four in the first year and seven in the second year out-yielded the midmonoculture yields and the increases ranged from 1 to 7.6 percent. Mixed stands were more stable than pure stands.The yield and yield components of the dwarf genotype HD 2160 scored less and those of the taller genotype C 306 scored higher in mixed stands. The semi-dwarf genotype Kalyansona yielded more with HD 2160 and less with C 306. Plant height but not high yielding ability conferred high competitive ability. With respect to competitive ability the three genotypes ranked as follows: C 306 > Kalyansona > HD 2160.The results illustrate the importance of intergenotypic competition in increasing crop production and reducing genotype-environment interactions. Such studies are important to agronomists as genotypes with high competitive ability can be useful to combat the weed problem. They are also important to plant breeders for predicting the fate of genotypes with low competitive ability in heterogeneous populations.     

Development of Stripe and Leaf Rusts in Wheat Cultivar Mixtures

Development of stripe and leaf rusts was studied for a two year period, from 1988 to 1990, in four cultivars of bread wheat planted in pure stands as well as in mixtures of different compositions in trials conducted at two different locations in Pakistan. The relative rate of disease development appeared to be slower in plots planted to cultivar mixtures as compared to that with pure stands of the same cultivars. Similarly, the area under the disease progress curve in cultivar mixtures was reduced, the reduction being proportional to the amount of the resistant component in the composite. Frequency of individual virulences in the parasite population also remained constant during the period of these studies. Differences in yield between plots with mixed populations and those with pure stands were also found to be statistically significant.     

Heterosis and mixing effects in barley under drought stress

Yield stability is one of the main breeding objectives in breeding for stress environments, such as the semi‐arid areas of Syria. The objectives of this study were to measure the effects of heterogeneity and heterozygosity on yield and yield stability by comparing doubled haploid lines (DHL) in mixed vs. pure stand (influence of heterogeneity) and F₂ populations vs. corresponding DHL mixtures (influence of heterozygosity). Six barley lines from two gene pools (LR = landraces, EL = experimental lines) were used to produce nine crosses (two LR × LR, three EL × EL, four LR × EL). The F₂ generation and eight DHL per cross were produced from each cross. The six parental DHL, nine F₂ populations, nine 8‐line mixtures and 72 DHL in pure stands were tested in five environments under drought stress in north Syria. The mean superiority of F₂ populations over DHL mixtures for yield traits across environments and cross combinations ranged between 7.5 and 10%. The effect of heterogeneity was small throughout. For grain yield, harvest index, 1000‐grain weight and plant height significant interactions between heterozygosity levels and environments were observed. The effect of heterozygosity for grain yield increased substantially from ‐1.2% in the highest‐yielding environment to 45.6% in the most stressful environment. Interactions between levels of heterozygosity and cross combinations were significant for most traits. F₂ populations were considerably more stable than DHL in pure stands, yet not as stable as DHL mixtures. It is concluded that heterozygosity is more important than heterogeneity in breeding for improved yield and yield stability under drought stress.     

Effects of Row Spacing, Support Plant Species and Support Plant Mixture Ratio on Seed Yield and Yield Characteristics of Hungarian Vetch (Vicia pannonica Crantz)

The effects of row spacing (17.5 or 35.0 cm), support plant species (barley or triticale) and the proportion of crops in mixtures (no support plant or support plant 20, 40 or 60 %, respectively) on the seed yield and yield characteristics of Hungarian vetch (Vicia pannonica Crantz) were investigated. Increasing the row spacing increased the seed yield of V. pannonica from 881.0 to 1248.0 kg ha^?1. On average, in a pure stand the seed yield of V. pannonica was 1141.0 kg ha^?1. In mixtures with barley and triticale, the seed yield of V. pannonica averaged 986.0 and 1143.0 kg ha^?1, respectively. In single mixed stands the seed yield of V. pannonica varied between 551.0 kg ha^?1 (60 % support plant barley) and 1603.0 kg ha^?1 (20 % support plant triticale). The yield advantage of V. pannonica in this triticale mixture was 40 % compared to the V. pannonica pure stand. With respect to the total yield in the mixture with 20 % triticale (1902.0 kg ha^?1) the yield advantage over the V. pannonica pure stand was as high as 65.1 %. In the mixed stands the number of seeds per pod and the thousand‐seed weight of V. pannonica were higher than in V. pannonica pure stands.     

Influence of Barley Scald Disease on Yield and Competition in Barley-Oat Mixtures

Field experiments with mixtures of barley and oats in southern Finland demonstrated a reduction in barley scald disease caused by Rhynchosporium secalis on upper leaves compared with that on pure barley stands. Mixtures lowered disease-induced yield reductions only slightly. In general, barley was not less competitive in the presence of disease than in its absence. Competitive relationships between the species were more important for yield determination than the direct influence of the disease. Over three years the yield increase of the mixtures compared with the means of pure components was close to 5%, whereas extreme conditions in one year led to a decrease of 11%. The role of competition for yield determination in barley-oats mixtures under disease conditions is emphasized.     

Compensation-Induced Yield Gains in Mixtures of Common Bean (Phaseolus vulgaris L.)

Over the course of two planting seasons field trials were conducted at the CIAT farm in Palmira (Colombia). The experiments included nine tropical bean lines grown in pure stands and seven mixtures obtained by combining these lines on the basis of equal proportions in seed number.
In the case of several pure lines, inferior seed quality led to low germination and uneven population densities, especially in the first planting season. Analogously, missing plants also occurred with the corresponding mixtures. A significant and positive relationship between mixing induced yield gains and patchiness in mixtures was found, indicating that intergenotypic compensation was a major determinant for mixture effects on yield. In common bean as a tropical subsistence crop seed quality is known to be erratic. It is concluded that compensation effects in mixtures of common bean may be expected to be of practical importance regarding the improvement of yield stability over seasons.     

Dry Peas (Pisum sativum L.) Grown in Mixtures with Faba Beans (Vicia faba L.)—A Rewarding Cultivation Alternative

Two semi-leafless pea cultivars and two highly lodging-resistant faba bean cultivars were grown in a series of replacement trials conducted on a highly fertile loam soil near Gottingen from 1992 to 1995. Cultivation as well as harvesting of mixed crops of both species proved possible. In spite of the delayed harvest of the mixed crop stands determined by the later pod ripening of the faba beans (varying between seasons from -f 5 to +21 days), noticeable seed losses due to pod shattering were not observed for the overripe peas. Standing ability of the peas was considerably improved by mixed cropping. Lodging began later and was less severe at harvest. Within mixtures with 50 % or more faba bean proportions at sowing (relative to the seeding rate of the pure stands) the peas, n i most cases, remained nearly upright until harvest and instead of the usual 'Plattlager', i.e., prostrate but upright top, frequently a more highly inserted 'Hohllager', i.e., upright but collapsed at higher nodes, was observed. In contrast to these remarkably synergistic effects for standing ability, such effects proved to be only small and nonsignificant for grain yield (−1.5–2.9%). Yield stability of the mixed crops was higher than that of the pure stands of both species. Estimates of the stability parameters, ecovalence, deviation from regression and environmental variance were at least halved. Yield proportions of peas and faba beans in the harvested seed of the mixtures did not correspond with the seed proportions at sowing. They differed considerably between seasons and cultivar combination. Higher yield proportions of faba beans were observed under wet and cold weather conditions and for the cultivar combination Victor/Disco, whereas higher yield proportions of peas were found under dry and warm weather conditions and for the cultivar combination Mythos/Baroness .     

Grain Yields of Perennial Grain Crops in Pure and Mixed Stands

Perennial grain crops were examined in pure stands and in various mixtures for their grain yield potential in, and suitability for, low‐input systems on marginal land. The investigation was based on field experiments conducted at two sites characterized by marginal environmental conditions in south‐west Germany in 1999 and 2000. The experiments included perennial species of rye (Secale cereale × Secale montanum), intermediate wheatgrass (Thinopyrum intermedium), lupin (Lupinus polyphyllus) and linseed (Linum perenne). In some stands, white clover (Trifolium repens) was included as an undersown intercrop for improved N supply. It was found that maturation of the species differed by up to almost 6 weeks, and consequently the threshability of most mixed stands was not satisfactory. Nevertheless, mixtures of either of the grasses (rye or wheatgrass) with lupin or undersown clover proved to be more suitable than pure stands in most cases, for measures such as grain yield, weed suppression and stability of grain yield over years. The grain yield harvested in the experiments in 1999 was low, reaching about 2.7 t ha^?1 with rye, and decreased for most species in 2000. If improved genotypes were available, perennial grain crops could provide an alternative for arable cropping and set‐aside areas under marginal conditions.     

Studies on Pure and Mixed Stands of Wheat and Barley Under Dryland Agriculture Conditions

Grain yield and yield components were studied in wheat and barley grown in pure and 2:1,1 : 1 and 1 : 2 mixed stands (Wheat : barley row ratio) as well as a 1 : 1 seed blend mixture under dryland agriculture conditions of the Indian Agricultural Research Institute, New Delhi. Wheat as well as barley produced significantly longer and more ears/m and higher grain yield in mixed stands than in their pure stands. The 2 : 1 wheat : barley mixed stand gave the highest grain yield; 35 % relative yield advantage over mid-monoculture yield. The 1 : 1 seed blend mixture was inferior to 1 : 1 (Wheat : barley row ratio) mixed stand.     

Yield and Yield Stability in a Balanced System of Widely Differing Population Structures in Zea Mays L.

With the view to study heterozygosity and heterogeneity as to their effects and interactions on yield and yield stability in maize, the following four types of population structure were formed from eight inbred lines: Homogeneous populations of homozygous plants (the eight inbred lines themselves); heterogeneous populations of homozygous plants (four blends containing four lines each); homogeneous populations of heterozygous plants (16 single crosses); heterogeneous populations of heterozygous plants (four blends containing four single crosses each, and four double crosses). This material was grown in three environments (years). At both levels of heterozygosity, the blends did not outyield the means of their respective components grown in pure stands. yield stability of the various structural groups was ranked differently by ecovalence and the mean square for deviations from regression. By transforming the deviation mean squares into coefficients of variation it was possible to reveal improvements of stability not only by heterogeneity but also by heterozygosity, and additionally, an interaction of both factors in the sense of a diminishing return.     

Agronomic performance and genetic shifts of genotype mixtures in peanut

Summary Little information is available on the genetic shifts that may take place when seed mixtures of peanut are grown over time. Five peanut lines with distinct seed coat colors, but similar plant types, pod yields, and maturities, were grown in pure stands for four years. A mixture with equal numbers of seed from each of the five lines was grown each year. All mixtures were subsampled and grown in subsequent years without reconstitution. Yield, 100-seed weight, and total sound mature kernel proportions from the mixtures were higher than the pure stand average of the five lines, but yield and total sound mature kernel proportions were lower than the average adjusted for individual line production. The best component line was significantly better than the mixture for pod yield, 100-seed weight, and proportion of sound mature kernels. Intergenotypic competition shifted the mixture composition to increase the proportion of one of the high yielding lines after only two years, while other high yielding lines in the test either maintained or decreased their original proportion. The two lowest yielding lines in pure stand also decreased in proportion over time.Contribution of the Florida Agricultural Experiment Station. Journal Series No. R-00550.     

Differential Competitive Ability and Growth Habit of Pearl Millet and Clusterbean Cultivars in a Mixed Cropping System in the Arid Zone of India

Dryland sustainable agriculture in the arid zone of India depends upon the choice of suitable cultivars for pure and mixed crop stands. Field experiments were conducted for two years to examine the response of two contrasting cultivars each of pearl millet (Pennisetum glaucum) and clusterbean (Cyamopsis tetragonoloba) in pure stands and in mixed pearl millet‐clusterbean stands. The differential response of cultivars of both crops to pure and mixed stands resulted in a significant genotype × cropping system interaction. Reduction in seed yield of both clusterbean cultivars was greater in mixed stands with tall and long duration pearl millet MH 179 than with medium statured and early maturing HHB 67. The degree of reduction was greater in Naveen, the branched clusterbean cultivar, than in RGC 197, the single stemmed cultivar. Mixing of pearl millet HHB 67 with medium duration clusterbean cultivar Naveen produced maximum pearl millet equivalent total yield. Higher land equivalent ratios (LERs) were also observed when clusterbean cultivars were mixed with early maturing and short statured pearl millet HHB 67.     

Co-adaptation between neighbours? A case study with Lolium perenne genotypes

Summary Co-adaptation between neighbouring genotypes of Lolium perenne was examined in a pot experiment over an 18 month period. Competitive interactions in pure and mixed stands were examined using 9 genotype pairs (established as neighbours from electrophoresis data), 7 from a 10 year-old, and 2 from a 40+ year-old pasture. Combinations of each pair of neighbours grown together, and each grown with 5 randomly chosen alien (non-neighbour) genotypes sampled from the same pastures were also assessed. Seven harvests of green matter produced above 20 mm were made, and 50–135 kg N ha^-1 was administered after each harvest. Data for dry weight at each harvest for pure and mixed stands, and Relative Yield Totals (RYT) were obtained for neighbours and aliens in each treatment.In 6 of the 9 pairs of neighbours grown as pure stands, the yields of neighbours did not differ significantly at the final harvest. In 8 of the 9 pairs grown as mixtures of pairs of neighbours, the yield of one of the neighbours was significantly reduced and one neighbour had been eliminated. Over the last 5 harvests, yields from (alien + neighbour) mixtures had the greatest dry matter yields. No suppression of one component by the other was recorded for this (alien + neighbour) combination.Based upon RYT estimates, neighbours did not yield more when grown together than when grown separately with aliens. There was no relationship between original genotype size in their fields of origin and dry matter yield in this experiment.This experiment provides no evidence for he evolution of co-adaptation between surviving neighbours in the two pastures from which these individuals were sampled. The genotypic architecture of pasture would seem to be the product of random and/or competitive exclusion of individual genotypes, rather than the evolution of co-adapted groups of individuals through time. The consequences of extreme depletion of genetic variation in the first two years after establishment, the very low rates of seedling establishment in established pastures, and the impact of the grazing animal would suggest that there is little likelihood of or potential for, co-adaptation evolving between neighbours.     

Inter-genotypic plant competition in wheat under single seed descent breeding

Summary The effects of inter-genotypic competition were studied in wheat under conditions of single seed descent in the glasshouse. Four genotypes were grown mixed in diallelic combinations of two, making six mixtures and four component (pure) stands. It was found that genotypic differences in most cases over-rode initial effects of phenotypic differences in seed size, and played a dominant role in determining what plants, if any, were eliminated from the mixtures.Low genotypic competitive ability accounted for 8 to 14% losses, in a single generation, in representation in the population through failure to produce at least one fertile tiller. Howerver, under single seed descent where artificial selection assures retention if even only one seed is produced, the loss of weak competitors was slower than it might have been under natural selection in the field.     

小麦混作冠层气象因子与群体产量关系的研究

本试验采用4个不同特点的小麦品种组成单作、双作、三作和四作群体,研究了它们冠层中的气象因子和群体产量表现及气象因子与群体产量间的关系。结果表明：混作产量都高于其组份品种单作的平均产量,最高增产达15％,混作成员单作的生产力水平越高,则它们混作的群体产量越高。气象因子中光与产量的关系最为密切．群体上、中、下三个层次的光强在各处理间的差异均达极显著水平,中部光强与小区产量呈显著正相关。混作可使群体结构得到改善,例二叶和倒三叶光合效率提高,此为混作增产的重要原因之一。     

Effekte der Sorten- (Weizen)und der Arten- (Weizen, Roggen) Mischung auf die Ertragsleistung krankheitsfreier Bestände

Effects of cultivar (wheat) and species (wheat, rye) mixtures on grain yield of desease-free stands
Cultivar mixtures (winter wheat) and species mixtures (winter wheat/winter rye) grown free from pests and diseases in different environments are compared to pure stands and analyzed with respect to inter-component relationships relevant for yield. All mixtures consisted from two components which were planted 1:1. Mixing effects (i.e. deviations from arithmetic mean of respective components grown in pure stands) on grain yield/ha and yield structure were dependent on components and growing conditions, particularly nitrogen availability. Under conditions suboptimal for one component, ontogenetic differences between components resulted in positive mixing effects on grain yield since the yield proportion of the accompanying partner increased more than proportionally. In several instances, yield proportions shitted without leading to mixing effects. In some experiments and/or mixtures mixing effects could not be observed at all.     

The Performance of Pea (Pisum sativum L.) and its Role in Determining Yield Advantages in Mixed Stands of Pea and Oat (Avena sativa L.)

On a brown warp soil (Fluventic Eutrochrept) near Goettingen, Germany, conventional leafed pea ( Pisum sativum L. cvs Messire and Bohatyr) and semileafless types (cvs Profi, Juno and Azur) were grown in mixed stands together with oat ( Avena sativa cvs Alf and Lutz) in substitutively designed experiments from 1995 to 1997. Oat was the dominant component. Crowding coefficients for oat averaged 7.4. No relationship could be detected between the crowding coefficient of oat and any yield advantage from the mixture. Crowding coefficients for pea varied substantially, between 0.1002 (Juno and Alf in 1996) and 0.2979 (Bohatyr and Alf in 1996). Crowding coefficients for semileafless pea cultivars were smaller than for conventional leafed types. The yield advantage of the mixture increased as the crowding coefficient of pea increased. The maximum yield increase for the mixture was achieved when the relative yield total (RYT)=1.17 or + 11 dt grain DM ha^–1 for mixtures of the long-strawed conventional leafed cultivars Bohatyr and Alf (in 1996). The crowding coefficients of pea were positively correlated with the level of symbiotically fixed N₂ in the mixed stands. When N₂ fixation with mixed cropping was about 30 kg N ha^–1, RYT was unity. Increasing symbiotic N₂ in the mixtures resulted in increasing yield advantages in the mixture. Short-strawed pea cultivars seem unsuitable for mixing with oat. Plant height of pea appeared to be more important than plant leaf type. Accordingly, mixtures containing the long-strawed semileafless pea cultivars Profi and Alf were more successful. It is concluded that increased competitiveness of the pea component in the mixture with oat entails increasing the level of symbiotic N₂ fixation including resource complementarity and thus yield advantage in the mixed stands.     

Genetic variability in herbage and seed yield in selected half-sib families of berseem clover, Trifolium alexandrinum L.

Berseem clover Trifolium alexandrinum L. is an annual forage legume commonly grown in pure stands and in grass mixtures in the Medi-terranean basin. Six populations were naturally cross-pollinated in 1990 and 1991 by a half-sib breeding method. In 1992 and 1993. 54 half-sib maternal plants, six original populations, and six advanced populations were field evaluated for forage (short cycle, harvest made at seven or eight inlernodes: long cycle, harvested at flowering) and seed yield (no forage harvest) in experiments at the Forage Crop Institute at Foggia, Italy (typical Mediterranean location). Genetic variability, narrow-sense herilability, genetic and phenotypic correlations among forage and seed yield component trails were investigated. The genetic variance in dry matter among maternal half-sib populations in short cycle was 51% greater than in long cycle. Narrow-sense heritabilily was 35% higher in short cycle than long cycle for dry matter and 26% higher for seed weight than seed yield. The magnitude of the genetic variance components and genetic correlations suggested that selection among plants of maternal half-sib populations would be more effective for improving dry matter in short than in long cycle harvests. The selection applied in the study was not effective for increasing seed yield per se however, the trait may be increased by selecting indirectly for seed weight.     

Adaptation and competition in mixtures of cowpea (Vigna unguiculata (L.) Walp.)

Summary Six pure lines and four mixtures were grown in six environments throughout lowland Papua New Guinea. There were no significant transgressive increases in the grain yield of mixtures above their pure line components, and mixture yields were adequately predicted by the mean of components. In yield stability it was found that the individual buffering of pure lines was of more importance than population buffering, and that the magnitude of population buffering varied with the particular combination of components. Competitive effects in all the mixtures were of the compensating type. Dramatic changes in mixture composition resulting from natural selection precluded their use in local agriculture. The outcome of competition in mixtures was strongly influenced by the growing environment, such that selective index was correlated to general fertility.     

Heritability of, and genetic correlations among, forage and seed yield traits in Ladino white clover

Sixteen white clover genotypes and their half-sib progenies were grown in pure stand. Each clover progeny was also grown in a mixed stand that also included three grass varieties of different species. Dry matter (DM) yield was measured over 2 years and seed yield at the second year. Competitive ability of clover families was defined as the ratio between mixed stand and pure stand for clover DM yield. Narrow-sense heritability from parent-offspring regression of pure stand data was high to moderate (h²0.50) for DM yield, seed yield and most of their components. Persistence as predicted by stolon density showed negative genetic correlations with seed yield (r_g= -0.70) and DM yield (r₂= -0.60), whereas seed yield and DM yield tended towards a negative correlation (r₂=0.45). Sizes of different vegetative or reproductive organs were generally correlated positively. Evidence was provided that the set of parent genotypes represents well the genetic variation available within the Ladino gene pool. The results highlighted the difficulty of combining relatively short-term DM yield, persistence and seed yield into a unique plant type. Differences in competitive ability emerged only in the second year, better ability being related mainly to petiole length among the traits assessed in pure stand.