Impact of annual legume 'break' crops on the yield and quality of canola in comparison with the impact on yield of wheat

This study investigated the yield, oil and protein content of canola in response to alternative single-season legume crops, and compared the yield response with that of wheat. Two field trials were conducted in consecutive years at Wagga Wagga on the South West Slopes of New South Wales. The soil type was Red Kandasol. The legume treatments were field pea and vetch managed for silage production, vetch managed for green manure, a mixture of aerial seeding clovers (Berseem, Arrowleaf, Persian: 6 : 3 : 3) managed for silage, hay or green manure, and field pea and narrow-leaf lupin managed for grain. There was one wheat treatment managed for grain. In terms of growing season rainfall, the rainfall-use efficiency of canola was low and similar to that achieved on farms (5.1 kg/ha.mm rainfall). Yields of canola were less than potential water-limited yields and the canola equivalent wheat yields were less than comparative wheat yields. The constraint(s) on canola yield meant that nitrogen-limited yield potential was not reached, and in contrast to wheat in adjacent plots, there was no relationship between canola seed yield and soil mineral nitrogen available during seedling establishment of canola. However, relative to other legume treatments, antecedent crops of the clover mix depressed yield and total oil content, particularly in one year. It was concluded that under field conditions that constrained canola productivity to that similar to on-farm productivity, wheat may make more efficient use of legume nitrogen than canola. Were canola to be used as the response crop, antecedent legumes of vetch or pea, rather than aerial seeding clover, would give higher potential yield and total oil content of the canola.