Farm Table says:
What is the problem?
Lucerne ( Medicago sativa L.) is finding an increasing role in dryland farming systems of the grain region of northern New South Wales and Queensland as a ley legume to improve soil fertility, grain yield, and quality. Residual herbicides play an essential role in weed management in crop rotations in this region, although their persistent residues may damage the susceptible crops sown following their use. Experiments were conducted to test the effects of application time of residual herbicides (winter and summer) on the survival and production of lucerne sown in the following autumn/winter
What did the research involve?
In July 1997, chlorsulfuron, metsulfuron-methyl, and triasulfuron were applied at Goondiwindi (trial 1), Roma (trial 2) and Warra (trial 3) in southern Queensland. In 1998, chlorsulfuron, metsulfuron-methyl, triasulfuron, clopyralid, and picloram + 2,4- D were applied in August, and metsulfuron-methyl, atrazine, imazapic, imazethapyr and picloram + 2,4-D were applied in December at Goondiwindi (trial 4) and Warra (trial 5). Herbicides were applied at the recommended and double rates to weed-free fallow plots. Soils were either sodosols (trials 1 and 4) or vertosols. Between spray application and planting, plots were hand weeded after significant rainfall events. Lucerne cv. Trifecta was sown during April/May 1998 in trials 1–3 and in June 1999 in trials 5–6. The establishment was good at all sites. However, trial 1 was abandoned after three months due to drought. The other trials received adequate rainfall.
What were the key findings?
The effect of the herbicides on initial seedling growth varied with herbicide, the rate of application and soil type. Seedling establishment was not affected. Reduced seedling vigor and phototoxicity were evident in some treatments. However, irrespective of seedling response, the Lucerne recovered with no subsequent reduction in dry matter production at any site at the first cut taken 5 months after sowing.
Herbicide residues adversely affected crops grown in sodosol soils more than in other soil types. Similarly, Churchett et al. (1999) found lucerne grown under glasshouse conditions was more affected by sulfonylurea herbicide residues in sodosol soil than in vertosol soil. In this study, only residues of chlorsulfuron, imazethapyr, imazapic and summer-applied metsulfuron-methyl, present in the sodosol soil produced severe phytotoxic symptoms in Lucerne. These symptoms occurred at only one site and at double the recommended rates, except for imazapic with severe symptoms at both rates.