DIFFERENTIAL TOLERANCE OF TRIFOLIUM-SUBTERRANEUM L (SUBTERRANEAN CLOVER) CULTIVARS TO BROADLEAF HERBICIDES .1. HERBAGE YIELD

The differential tolerance of 7 Trifolium subterraneum (subterranean c lover) cultivars to 5 broadleaf herbicide treatments applied at 2 rate s was examined at 2 sites over 2 years. The herbicide treatments and t he rate of active ingredient applied (kg a.i./ha) were 2,4-DB (0.8, 1. 6), MCPA (0.5, 1.0), bromoxynil (0.28, 0.56), MCPA (0.15, 0.3) + terbu tryn (0.275, 0.4125), and MCPA (0.25, 0.375) + diuron (0.25, 0.375). T he herbage yield of all cultivars at 30 and 60 days after herbicide ap plication (DAA) was suppressed by all herbicides except 2,4-DB at 60 D AA. There was marked variation in cultivar tolerance, with Trikkala be ing the most tolerant to all herbicides. Seaton Park was the most sens itive to MCPA + terbutryn, MCPA + diuron, and bromoxynil, while Karrid ale was the most sensitive to MCPA. Log(10)(sprayed yield) - log(10)(u nsprayed yield) was an appropriate measure of relative tolerance of cu ltivars to herbicide. While herbage yields and yield reduction due to the herbicides varied between sites and years, the effect of the herbi cides and the relative responses of the cultivars were similar across years and sites. Herbage yield meaned over the 2 herbicide rates was r educed by 16-71% at 30 DAA depending on herbicide and cultivar, and by 4-45% at 60 DAA. A mixture of MCPA + terbutryn or MCPA + diuron cause d the largest reduction in herbage (47-71%) at 30 DAA, while MCPA and 2,4-DB caused the smallest (16-56%) reduction. Neither the maturity ra nking of the cultivar nor the cultivar vigour measured at 30 DAA influ enced sensitivity to the herbicides. The rate of recovery (kg DM/ha.da y) by the cultivars at 30-60 DAA was greatest in the MCPA, 2,4-DB, and bromoxynil treatments (80-120 kg DM/ha.day) and least in the MCPA + d iuron treatment (71-84 kg DM/ha.day) and was not related to maturity r anking. Doubling the rate of herbicide increased herbage yield suppres sion and resulted in slower growth rates but did not affect seedling s urvival. The magnitude of the reduction in herbage yield has important implications for choice of herbicide and needs to be balanced against the likely loss due to weed competition. The identification of signif icant differential cultivar tolerance suggests that worthwhile improve ments in tolerance can be achieved through plant breeding or selection .