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The impact of grass control on merino lamb production in the cereal belt of south Australia

D.L. Little, E.D. Carter and A.L. Ewers

Department of Agriculture, Turretfield Research Centre, Rosedale SA 5350
University of Adelaide, Roseworthy Campus, Roseworthy SA 5371

Grass control in pasture has been promoted because of increased cereal yields resulting from control of root diseases hosted on a number of pasture grasses. However, control of grasses also has implications for animal production, one of which is reduced grass seed problems.


Six adjacent grassy pastures (3.63 ha each) dominated by barley grass, Hordeum leporinum, were divided into three paired replicates. One paddock of each pair was sprayed with 350 ml/ ha of Fusilade 212'(212 g/l fluazifop-P [butyl ester] a.i.) on 10 September 1990. All paddocks were stocked with 37 woolly Merino lambs (10.2 lambs/ha) on 28 September. The lambs were weighed three times up to day 42 of grazing after which they were shorn (day 47) because of grass seed problems in the unsprayed treatments. The lambs remained on the pastures and were weighed weekly for a further two weeks when they were valued by an independent assessor and slaughtered (day 69). Carcasses and pelts were assessed.

Results and discussion

The herbicide spraying was very effective in converting pastures of >90% grass to pastures of >50% legume with the balance being herbs and dead grass. However, available dry matter was reduced in the sprayed treatments (1.1 vs. 3.6 t/ha for sprayed vs. unsprayed).

Before the appearance of grass seeds, lambs in both treatments gained weight but by day 28 the sprayed treatment had a 1.8 kg/hd advantage. Grass seeds began penetrating fleeces in the unsprayed treatments on day 35 and by day 42 lambs had lost condition. After shearing, the grass seed problems were greatly reduced and the lambs on the unsprayed treatments began to gain weight, indicating that the previous weight loss was primarily due to grass seed irritation and not to reduced nutritive value of the pasture. The percentage of lambs affected by grass seeds in the eyes was 0.9 and 57.7 for the sprayed and unsprayed treatments respectively. The lambs grazing the sprayed treatments continued to gain weight throughout the trial and had a 6.2 kg/hd advantage by day 63.

Mean carcass weights were 16.1 kg and 13.7 kg and mean carcass values were $7.57 and $1.64 for the sprayed and unsprayed treatments respectively. Pelts from the unsprayed treatments were heavily infested with grass seeds but only light seed damage was apparent in pelts from the sprayed treatments. Mean wool yields per animal were similar (2.26 kg and 2.22 kg for the sprayed and unsprayed treatments respectively) but vegetable matter contamination in wool from lambs in the unsprayed treatments was approximately doubled. This resulted in a 16% reduction in wool value per animal from the unsprayed treatments.

We have shown that, providing there is a sound legume base in a pasture, herbicide spraying of barley grass before seed maturity results in improved pasture quality, elimination of grass seed problems but decreased pasture availability. Grass control ensures heavier lambs and allows greater flexibility in time of selling. Carry-over lambs will have better growth into the dry summer period and legume dominance will ensure higher soil N in the following year.

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