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The effect of broad-leaved herbicides on the pattern of water use in trifolium subterraneum

G.A. Sandral and B.S. Dear

NSW Agriculture Agricultural Research Institute, Wagga Wagga NSW 2650

A range of herbicides are applied to Trifolium subterraneum based pastures to control broad- leaved weeds. The objective of weed removal is to reduce competition for light, nutrients and water. However, these herbicides have been shown to depress the growth of the legume. It is therefore possible that the water use pattern of the legume may be altered as a result of either depressed growth rates, smaller leaf areas, or direct effects on stomata control. This poster describes the results of an experiment which examined the effects of four herbicides on the pattern of water use in two cultivars of T. subterraneum.

Methods

A glasshouse experiment was conducted with three sowing times (3 April, 27 April and 21 May), two cultivars of T. subterraneum (Karridale and Trikkala). Herbicide treatments included an unsprayed control, bromoxynil (n-octanoyl ester) at 0.28 kg a.i./ha, MCPA (dimethylamine salt) at 0.5 kg a.i./ha and a mixture of MCPA + terbutryn at 0.15 and 0.275 kg a.i./ha. Each pot contained five plants. Plastic beads were added to the pots to minimise evaporation. Pots were watered to field capacity daily and the quantity of water added, recorded. The experiment was a randomised block design with four replicates. Results from the two sowing times are presented for plants sprayed at three to four trifoliate leaves and meaned over cultivars.

Results and discussion

Total plant water use over the 40-day period following spraying was reduced by all herbicide treatments at both sowing times. Plant water use at 20, 30 and 40 days after the application of the herbicide was also significantly lower than the unsprayed controls for both sowing times (Fig. 1). This experiment highlights the significant decrease in water use that can occur following the application of these herbicides. Water conserved may then be available in spring when moisture stress frequently coincides with seed filling. These effects on water use need to be considered when interpreting the results of herbicide tolerance screening trials. Recent unpublished work by the authors suggests reduced transpiration area and increased stomatal resistance are the most likely causes of lower water use.

Figure 1. Reduction in water use (mls/pot/day) by Trtfolium subterraneum sown on (a) 3 April and (b) 27 May for 40 days following herbicide application.

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