DIFFERENTIAL TOLERANCE OF TRIFOLIUM-SUBTERRANEUM L (SUBTERRANEAN CLOVER) CULTIVARS TO BROADLEAF HERBICIDES .2. SEED YIELD AND QUALITY

The effect of broadleaf herbicides on seed set by Trifolium subterrane um (subterranean clover) cultivars was examined at 2 sites (Wagga Wagg a and Canowindra) over 2 years. Five commonly used herbicide treatment s (bromoxynil, MCPA, 2,4-DB, MCPA + terbutryn, MCPA + diuron) were app lied at 2 rates to 7 cultivars of subterranean clover. Significant sit e x cultivar x herbicide interactions were observed. Seed yields were either unaffected or depressed by up to 66% at the higher rainfall sit e (Canowindra), whereas at the lower rainfall site (Wagga Wagga) some herbicide x cultivar combinations showed increases in seed yield up to 115%. The increase in seed yield was greater at the lower herbicide r ate. The variation in seed yield with herbicide treatment was largely a result of a change (P<0.001) in the number of seeds set (R = 0.94 at Wagga Wagga; R = 0.85 at Canowindra). Seed size was also correlated ( P<0.001) with seed yield at both sites but explained less of the varia tion (R = 0.23 at Wagga Wagga; R = 0.47 at Canowindra), The reduction in herbage yield as a result of herbicide application was a poor indic ator of the subsequent seed yield response at both sites. Cultivar Tri kkala was consistently most tolerant to the herbicide treatments, show ing either no change in seed yield at the wetter site or large increas es in seed yield at the lower rainfall site. In contrast, the seed yie ld of Dalkeith was depressed by 2,4-DB at both sites in both years by 39-66%. Increases in seed yield, which were most pronounced in cultiva rs of midseason maturity (Trikkala, Junee, Seaton Park) and least in l ater maturing cultivars (Karridale, Glare) and in the very early flowe ring Dalkeith, were attributed in part to a water-saving effect of the herbicide treatments. Cultivar maturity ranking was negatively correl ated (P<0.001) with seed yield (R = -0.73 at Wagga Wagga; R = -0.45 at Canowindra). The germination percentage of seed produced by the culti vars was unaffected by herbicide treatment, although the number of abn ormal radicles formed by germinating seed was higher in the 2,4-DB and MCPA treatments.