A physiological study of weed competition in peas (Pisum sativum L.)

Abstract

Peas dominate New Zealand grain legume production and they are a major export crop. However, weeds are a major problem particularly under organic production, where the use of synthetic chemicals is prohibited. To address this limitation, a research program to study weed control in peas was done to provide both conventional and organic farmers a sustainable weed management package. This was done through three field experiments over two growing seasons, 2006/07 and 2007/08. Experiment 1, (2006/07) evaluated the effect of 50, 100 and 400 plants m² on crop yield, and weed growth of Aragon, Midichi or Pro 7035 with and without cyanazine. Experiment two explored the physiology of two pea genotypes, the leafed (Pro 7035) and the semi leafless (Midichi) sown at three dates. A herbicide treatment was included as a control. In the third experiment Midichi, was used to investigate the effect of different pea and weed population combinations and their interaction on crop yield and weed growth. All crops were grown at Lincoln University on a Templeton silt loam soil. In Experiment one, herbicide had no effect on total dry matter (TDM) and seed yield (overall mean seed yield 673 g m²). There was also no significant difference in mean seed yield among the pea genotypes, Aragorn, Pro 7035 and Midichi, (overall mean, 674 g m²). The lowest average seed yield, 606 g m² was from 400 plants m² and the highest, 733 g m², from 50 plants m², a 21% yield increase. A significant herbicide by population interaction showed that herbicide had no effect on seed yields at 100 and 400 plants m². However, cyanazine treated plots at 50 plants m² gave 829 g m² of seed. This was 30% more than the 637 g m², from plots without herbicide. In Experiment 1 pea cultivar and herbicide had no significant effect on weed counts. In Experiment 2 the August sowing gave the highest seed yield at 572 g m². This was 62% more than the lowest yield, in October. Cyanazine treatment gave a mean seed yield of 508 g m², 19% more than from unsprayed plots. There was a significant (p < 0.05) sowing date x genotype interaction which showed that in the August sowing genotype had no effect on seed yield. However, in September the Pro 7035 seed yield at 559 g m² was 40% more that of Midichi and in October it gave 87% more. Weed spectrum varied over time. Prevalent weeds in spring were Stachys spp, Achillea millefolium L., and Spergular arvensis L. In summer they were Chenopodium album L., Rumex spp, Trifolium spp and Solanum nigrum L. Coronopus didymus L., Stellaria media and Lolium spp were present in relatively large numbers throughout the season. In Experiment 3 seed yield increased significantly (p < 0.001) with pea population. Two hundred plants m² gave the highest mean seed yield at 409 g m² and 50 plants m² gave the lowest (197 g m²). The no-sown-weed treatment gave the highest mean seed yield of 390 g m². This was due to less competition for solar radiation. There was no difference in seed yield between the normal rate sown weed and the 2 x normal sown weed treatments (mean 255 g m²). It can be concluded that fully leafed and semi-leafless peas can be sown at similar populations to achieve similar yields under weed free conditions. Increased pea sowing rate can increase yield particularly in weedy environments. Early sowing can also increase yield and possibly control problem weeds of peas (particularly Solanum spp), which are usually late season weeds. Herbicide can enhance pea yield but can be replaced by effective cultural methods such as early sowing, appropriate pea genotype and high sowing rates.

Additional key words: Pisum sativum L., semi-leafless, fully leafed, cyanazine, pea population, weed population, sustainable, TDM, seed yield, weed, weed counts, sowing date, weed spectrum, seed rates.