The effect of intercropping systems and cultivar mixtures on weed and disease suppression in organically managed spring wheat

Without the chemical supports of high-input agriculture, weeds and diseases can become serious hindrances to successful organic wheat production. Diversifying the cropping system through strategies such as intercropping and cultivar mixtures may provide effective suppression of weeds and diseases and thereby improve grain yield and quality. An intercropping system trial and a cultivar mixture trial were evaluated at Carman, Manitoba and Clearwater, Manitoba in 2004 and at Cayman, Manitoba in 2005 to determine their impact on weed and disease levels as well as biomass and grain yield. The study consisted of two trials, the first of which examined intercrop mixtures comprised of three representative systems. The first system was wheat and other cereal mixtures, which included wheat and oats, wheat and barley and wheat and spring rye. The second system was wheat and other seed intercrop mixtures and included wheat and flax, wheat and field peas and wheat and oriental mustard. The third system was wheat and cover crops, which included wheat and red clover, wheat and hairy vetch, and wheat and annual ryegrass. In the cereal intercrop system, wheat-barley was the highest yielding and most profitable of the cereal treatments, with additional benefits in weed suppression. Within the wheat and other seed intercrop system, wheat-flax could be profitable but flax reduced the wheat crop in all site-years, and underwent crop failure due to frost damage in one site-year. Wheat-field peas reduced disease levels in the wheat crop in all three site-years but yields and net returns were only as profitable as the full-rate monoculture in one site-year. Wheat-oriental mustard was profitable in all site-years even with the oriental mustard being largely decimated by flea beetles in two of three site-years. In the cover crop system, wheat-red clover had little variation in yield or profit from the half-rate wheat treatment in all site-years, but tended to lower disease levels. Wheat-hairy vetch could boost wheat yields and profitability to levels comparable to full-rate wheat, but it also could overwhelm the wheat crop and cause a net loss in a site-year with excess precipitation. Wheat-annual ryegrass could also overwhelm the crop in a wet site-year, but it also could significantly reduce weed levels. The second trial of the study examined the heritage and modern varieties of Red Fife, Marquis, AC Barrie and 5602HR within monocultures and cultivar mixtures under organic management. This trial included treatments of all possible combinations of the varieties, as well as sole variety treatments. There was little variation among the treatments, which demonstrates the ability of cultivar mixtures to stabilize yields. However, amongst the sole variety treatments, 5602HR and Red Fife had higher yields than AC Barrie or Marquis, with 5602HR having significantly higher yields than many other treatments in conditions of flooding stress. Red Fife and mixtures containing Red Fife also had lower levels of disease than the other treatments. Both intercropping systems and cultivar mixtures present examples of crop mixtures which, while increasing diversity, can also provide benefits of weed/disease suppression as well as stabilizing or increasing grain yield and profitability.