Effect of time of application of MCPA and bromoxynil in relation to flowering on subterranean clover seed yield and quality

Abstract

The importance of stage of phenological development and cultivar maturity ranking in influencing the seed yield response of subterranean clover (Trifolium subterraneum L.) to broadleaf herbicides was examined in a field study in southern New South Wales. The herbicides, bromoxynil (200 g a.i./L) at 1.4 L/ha and MCPA (500 g a.i./L) at 1.0 Wha, were applied 6 (TI) and 3 weeks (T2) before flowering, and at 5% flowering (T3) to 7 cultivars ranging from early to late flowering. The stage of phenological development at herbicide application, the maturity ranking of the cultivar and the herbicide applied, all influenced seed set. Application of bromoxynil at T1 did not reduce seed yield of any cultivar but increased seed yield of 4 cultivars by up to 30%. MCPA applied at the same time reduced seed yield in 1 cultivar by 26% and increased yields in 4 others by up to 57%. Application of bromoxynil at T3 caused reductions in seed yield in 3 of the 7 cultivars. MCPA applied at this time was more damaging than bromoxynil with all cultivars showing large seed yield reductions ranging from 22 to 64%. Despite large reductions in herbage yield as a result of some herbicide treatments, there was only a low correlation (r = 0.3, P<0.01) between herbage yield at full flower and seed yield. Our findings suggest that the reduction in herbage yield and change in canopy structure as a result of the early application of herbicides stimulated seed production. Seed number was the primary yield component responsible for the change in seed yield with a high correlation (r = 0.86, P<0.01) between these 2 parameters. Seed size was relatively insensitive to herbicide treatment but varied markedly with cultivar. Germination percentage of seed produced by swards treated with bromoxynil was unaffected, even when applied at T3, whereas MCPA lowered germination percentage of almost all cultivars if applied within 3 weeks of the commencement of flowering. The maturity ranking of the cultivars was highly negatively correlated with percentage seed yield loss (r = -0.80, P<0.01) when either herbicide was applied at T3, the seed yield of the early flowering cultivars being most depressed by the herbicides. Length of recovery period between herbicide application and flowering was shown to be very important in influencing seed yield, the effect being modified by both herbicide and cultivar maturity.