DETRIMENTAL EFFECTS OF VOLUNTEER Brassica ON PRODUCTION OF CERTAIN CEREAL AND OILSEED CROPS

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Abstract
Variable amounts (up to 1.04 t dry wt ha−1) of herbage from seedlings of cultivars from four Brassica species (B. campestris L., B. napus L., B. hirta Moench and B. juncea (L.) Coss.) and straw and chaff (5 t ha−1) of B. campestris were incorporated into field plots to investigate the effect on subsequent stand establishment and seed yield of barley (Hordeum vulgare L.), oats (Avena sativa L.), wheat (Triticum aestivum L.), flax (Linum usitatissimum L.) and rapeseed (B. campestris). The amounts of plant material incorporated were considered representative of straw and chaff which might be deposited at harvest, or herbage which could originate from postharvest volunteer germination. Brassica herbage reduced stand establishment and seed yield of the oilseeds to a greater extent (65 and 53%, respectively) than that of the cereals (17 and 23%, respectively). Among the cereals, barley was the most affected, while oats did not show significant reductions of stand or yield. Generally the type of Brassica used to produce herbage did not influence the reduction of stand establishment or seed yield. Incorporation of straw and chaff of rapeseed in some instances resulted in reduced plant stands but did not show deleterious effects on seed yield, and in one case resulted in an increase in yield. For comparison, the effect of incorporating barley herbage on stand establishment and seed yield of barley and flax was also investigated. Incorporation of barley herbage also resulted in reduction of stand establishment (21%) and seed yield (26%) of subsequent crops, although its effect on stand establishment was less pronounced than for Brassica herbage (44%). It is recommended that a cereal, in particular oats, follow rapeseed or mustard in the crop rotation in order to avoid losses from deleterious effects.Key words: Brassica campestris, Brassica napus, Brassica juncea, Brassica hirta, rapeseed, mustard, crop residue, phytotoxicity