The effect of site on competition and yield advantages of mixtures of barley and oats

Competition between six-row barley cv. Agneta and oats cv. Veli, and yield advantages of the mixtures were evaluated in a replacement series field experiment. The experiment was situated in a sloping area. Althoughbarley was lower yielding (grain yield) than oats when the components were grown in monoculture, barley was dominant in all mixtures irrespective of the site. The competitive ability of barley decreased from the less productive site (top) to the more productive site (bottom). The improved competitive ability of oats was likely due to the faster early growth of


INTRODUCTION
In recent years there has been increasing interest in the use of mixtures in practical culti- vation.Approximately 50 % of the barley and oat grain produced in Ontario is from mixtures of the two species (300000 ha) (Fejer et  al. 1982).Some possible advantages of mixtures are greater yield , greater stability in yield over different environments and lower incidence of disease.The results of earlier studies of barley-oats mixtures have been discussed recently by Jokinen (1991 a,b).In general, mixtures have slightly higher yields than the mean of their components, and even overyielding may occur.The results of some studies also suggest that the relative yield to- tal (RYT) may be greater than one, suggest-ing that a larger area of land is needed to produce the same yield of each species with monocultures than with a mixture.
Although the competition between plants and the yield advantage of mixtures are differ- ent aspects of mixture research, competition has an profound effect on the yield advantage of mixtures.Because plants are competing for limiting environmental resources, the environ- mental variability may be expected to affect the relationship between plants.According to the resource competition theory developed by Tilman (1982) the differences in the availabil- ity of two resources such as nitrogen and light may have significant effects on the competitive relationship between plants.Thus plants growing in small experimental plots on uni- form sites may be too limited to demonstrate the advantages of the versatility of mixtures.
The aim of the present experiment was to evaluate the competition between barley and oats and the yield advantage of the mixture on a sloping area.

MATERIALS AND METHODS
The field experiment was carried out in 1985 on the Kotkaniemi Experimental Farm of Kemira Oy in southern Finland (60°22'N, 24°22'E).Six-row Agneta barley and Veli oats were seeded separately (500 seeds/m 2 ) and in an equal mechanical mixture (250/250) (replacement series).The general characters of the cultivars are described elsewhere (Jokinen

b).
A sloping area with three sites (top, middle and bottom) was employed.The soil was finer fine sand with pH 5.7.A split-plot design (sites in main plots and genotypic composition of stand in subplots) was used with four blocks.The plot size was 30 m 2 (3 m x 10 m) with rows spaced 12.5 cm apart.The rows of subplots were laid along the slope with the subplots of each site being adjacent.The fer- tilizer was granular NPK (N 16%, P 7%, K 13%), the amount was adjusted to 80 kgN/ha, and the placement method was applied.The sowing date was 17 May.The crops were kept free of weeds by one application of the herbi- cide Actril S (2-3 liters/ha mixed with 300 liters of water) containing Mcpa (235 g/1), dichlorprop (184 g/1), ioxynil (38 g/1) and bromoxynil (24 g/1) at the time of shoot emer- gence.
The number of plants in each plot was de- termined by counting the number of seedlings in four randomly selected 1-m-long rows/plot about three weeks after sowing and before the initiation of tillering.At maturity the entire area of each plot was harvested (28 August) and the grain yields were determined (kg/ha at 15% moisture content).From each mixture yield a 50 g sample was taken for determina- tion of the seed yield of the barley and oats components.The separated samples of each mixture as well as samples of each pure stand yield were used for determination of the protein content (%) of the grain by the Kjeldahl method (500 mg with two subsamples) and 1000-grain weights (g) (3 x 100 seeds/sample).
Relative yield (RY) and relative yield total (RYT) were calculated according to the methods of de Wit and van den Berg (1965).Also relative protein yield and relative protein yield total were determined.Competitive ra- tio (CR) was determined according to the method of Wiley and Rao(1980).The mean yield/area of four replications was calculat- ed before computing the indices.
The grain yields, 1000 grain weights, the protein content of grain and the protein yields were subjected to analyses of variance for split-plot design (Steel and Torrie 1980).Mean separation was accomplished by Tukey's honestly significant difference test (HSD) (P = 0.05) (Steel and Torrie 1980).

RESULTS AND DISCUSSION
Early development of the plants Barley seed sown at the top of the hill emerged first, a few days earlier than oats.In the middle of the slope and especially at the bottom there were no differences in the time Table 1.The grain yields, relative grain yields (RY), relative grain yield totals (RYT), actual grain yield/expected grain yield (A/E) and competitive ratio of barley to oats (CR) at different sites.BB = barley yield in pure stand, OO = oats yield in pure stand, M = mixture yield, BM = barley yield in mixture, OM = oats yield in mixture, RYB = barley relative yield, RYO = oats relative yield.Grain yield means within site rows, grain yield means in the average column and grain yield means in the average row followed by the same letter are not significantly different at the 5 % level (HSD test). of emergence between the two species.This might be due to the greatest moisture of the soil being at the bottom of the slope according to observations.At the beginning of the growing season the density was approximately the same (0.95 -1.05) as expected (data not given).
The first leaves of barley were longer and wider than those of oats according to obser- vations.This indicates a larger area of photosynthesis during the early stages of growth and development.In other studies, where quantitative measurements have been done, Table 2.The effect of site and genotypic composition on the thousand grain weight(g) of barley and oats.The difference between treatments analysed statistically for each species.BB = barley in pure stand, 00 = oats in pure stand, BM =barley in mixture, OM = oats in mixture.Me- ans in the average column and means in the averagerow, followed by the same letter are not significantly different at the 5% level (HSD test)

Competitive ability and grain yield advantage
The reader should observe that barley was more productive in mixtures, both in terms of absolute and relative yields, and was also more competitive than oats as determined by com- petitive ratio irrespective of the site (Table 1).Thus the magnitude of the monoculture yield was less important in determining the domi- nance of the component in mixtures, as shown also in other studies (Spitters 1979, Alex-  ander et al. 1986, Jokinen 1991 b).
There were no significant differences in grain weight whether the species were grown alone or in mixtures (Table 2).In other studies of barley-oats mixtures (Syme and Bremner   1968, Jokinen 1991 b) it was shown that the number of ears per plant and the number of grains per ear varied more than grain weight in circumstances where differences in monoculture and mixture yields were observed.This suggests that the species competed for the resources, and the dominant- suppression relationship was determinend at earlier stages than during grain filling.
The environment was the least favourable at the top (p<0.05)and the most favourable at the bottom of the hill for grain production (p<0.05)(Table 1).This is an indication of gradual increase and thus improved availabil- ity of soil resources through the environmen- tal gradient.
In addition to the main effects there was also interaction between the site and the stand (p<0.05).Barley grown in monoculture was less productive on the top and in the middle of the slope compared with other stands.The mixture overyielded in the middle and at the bottom of the slope.However, the difference between the yield of the mixture and the yield of the oats grown in monoculture (about 4%) was not statistically significant.
Irrespective of the site, both relative yield totals and the ratio of actual and expected yields were greater than one and thus fulfilled the requirements of using a mixture even un- der variable field conditions (Table 1).Thus from a practical point of view, it might be more advantageous to cultivate these varieties in mixtures than in monoculture.
Both competitive relationship between com- ponents and the monoculture yields affected the total grain yield of the mixture (Table 1).
When the environment was considerably less favourable for the aggressor grown alone than for the subordinate grown alone, the aggres- sor being the least productive component grown in monoculture, the mixture did not overyield.Thus the later the growing season as environmental stresses such as decreased availability of nutrients and water or some pathogens reduce the yield of the aggressor, the poorer are the possibilities for the subor- dinate to compensate.This concerns especially the cases where the share of the space occurs during the very early stages of the growth.In several mixtures of barley and oats a rather severe disease caused by Rhyncosporium secalis had only a slight effect on the competitive relationship, the grain yield of the mixture de- pending more on the yield level of barley grown in monoculture (Karjalainen and Jokinen unpubl.).Also Alexander et al.
( 1986) found that the relative competitive abil- ities of wheat cultivars were not reversed when disease was present, the susceptible cultivar being a stronger competitor and yielding less than the resistant cultivar in monoculture.

Competition for resources
The monoculture grain yield of barley increased more than that of oats when the production conditions were improved (Table 1), agreeing with the previous results of Jokinen (1991 a, b) and Vermeulen (1991).In the present experiment, the grain yield of oats, when grown alone, was greater than that of barley in low productivity conditions, which was also observed by Vermeulen (1991) and  Jokinen in 1983 (1991 a).Thus these results suggest that barley responds more to soil resources, and oats is better adapted to a low level of soil resources.The earlier emergence of oats at the bottom than at the top had a positive impact on the competitiveness of oats (Table 1).This might be due to the increased availability of light and subsequent faster growth of oats at the bottom.Thus combined with earlier studies of barley cv.Agneta and oats cv.Veli in 1983 (Jokinen 1991 a) the present results suggest that barley cv.Agneta is a superior competitor for light and oats cv.Veli a superior competitor for soil resources.This suggestion is based on the resource com- petition theory (Tilman 1982) which predicts that the species which is the superior compet- itor for a resource will become less dominant as that resource is added.It will become increasingly dominant as the resource for which it is inferior competitor is added.Thus, it is important to note that in the mixtures of the present trial the increase of light for oats had a stronger influence on the competitive rela- tionship than the influence of the increased soil resources on barley.The competition the- ory also suggests that each species should be a superior competitor for a particular point along the nutrientdight gradient.Changes in relative availability of these resources should lead to changes in the composition of the com- munity, as occurred earlier (Jokinen 1991 a)   as well as in the present experiment (Table 1).
These results show the complexity of competition for resources even in slightly different environments.They also show how the addi- tion of soil resources might increase the yield per se but might also unpredictably modify the competitive relationship between two species because of an interaction between different growth factors.
The species approached a stable coexistence from top to the bottom but did not achieve it, barley still being in all cases a winner.How- ever, these two genotypes can stably coexist in an agricultural environment at a particular point along the productivity gradient (Jokinen 1991 a).According to Vandermeer (1989   p.69), if two species coexist, it is likely that they do so because their niches do not over- lap sufficiently.Also the results of relative yield totals (RYT> 1) in the present trial (Ta- ble 1) suggests that there might occur resource partitioning between components as shown earlier by Jokinen (1991 a,b).
Additional experiments are, however, need- ed to confirm that these barley and oats vari- eties do exhibit the suggested tradeoffs.In the next experiments precise measurements like the phytomass accumulation and availability of growth factors in the course of competition along the productivity gradient should also be done.The reciprocal model should be preferred to the de Wit model (see for exam- ple Jokinen 1991 a).
Protein content and protein yield The protein content of grain yield of oats grown in mixture was the highest (p<0.05),whereas the lowest protein content (p<0.05)occurred in the grains of barley grown in mixture (Table 3).In low light environments, plants such as oats in the present mixtures might accumulate high nutrient concentra- tions, but have low carbohydrate contents (Bloom et al. 1985).Along the gradient the protein content decreased, showing a negative correlation between the grain yield and pro- Table 3.The effect of site and genotypic composition on the protein content (%) of the grain yields, BB = barley in pure stand, 00 = oats in pure stand, BM = barley in mixture, OM = oats in mixture, M = mixture.Means in the average column and in the average row followed by the same letter are not significantly different at the 5% level (HSD test).tein content common in cereals.

Site
The protein content of the mixture was low- er (p<0.05)than that of oats and about the same as that of barley.Because the dominant component had a lower protein content than the subordinate and the dominant grown in monoculture was lower yielding, the mixture did not overyield in respect to protein yield (Table 4).
The ratio of the actual and expected pro- tein yield of the mixture was close to one.However, the results of relative yield totals of protein yields indicated that the use of mixture might be more advantageous than monocultures with the magnitude of the advantage depending on the environment (Table 4).This agrees to a certain extent with the previous results (Jokinen 1991 b).

Table 4 .
Protein yields (kg/ha), relative protein yields (RY), relative protein yield totals(RYT)and actual protein yield/expected protein yield (A/E) at different sites.For abbreviations see Table . Means in the average column and means in the average row followed by the same letter are not significantly different at the 5% level(FISD test).