Effects of initial sowing rate and subsequent grazing management on the growth and clover content of irrigated white clover - perennial ryegrass swards in northern Victoria

Abstract

Two experiments were conducted over 3 years. One was of factorial design involving 2 sowing rates of white clover (Trifolium repens L. cv. Haifa; sown at 8 or 3 kg/ha) and perennial ryegrass (Lolium perenne L. cv. Grasslands Nui; sown at 5 or 15 kg/ha) grazed by dairy cows at 2 frequencies [frequent (2–3 weeks in spring/autumn) and infrequent (4–6 weeks in spring/autumn)] and 2 intensities [hard (residual rising plate meter heights of less than 4 cm) and lax (residual rising plate meter heights of more than 5 cm)]; and the second was a regression design involving 5 sowing rates of white clover and ryegrass ranging from pure clover to pure ryegrass (sown at 10/0 through to 0/20 kg/ha), all grazed frequently and at a hard intensity. The hypotheses tested were that (1) pure white clover swards would be at least as productive as those that contained ryegrass, and (2) more frequent grazing would result in greater quantities of DM removed, while hard grazing would maintain a higher clover content. In general, the hypotheses were confirmed. Over the 3 years of the experiments, pure white clover swards were at least as productive as mixed swards in a situation where no nitrogen fertiliser was applied. In the first year, the amount of DM removed declined (P<0.05) as the ryegrass sowing rate increased, but by year 3, the pure clover treatment out-yielded the other treatments. Except for the first year, frequent grazing resulted in more (P<0.05) DM removed than did infrequent grazing. Frequently grazed swards also had higher daily net photosynthesis after grazing than did the swards in infrequently grazed treatments, and achieved maximum levels of photosynthesis more quickly. There was no difference in photosynthesis rate, despite significant differences in clover content, between sowing rate treatments, regardless of grazing management. Initial sowing rate had a large effect on clover content in year 1, but by year 3, most of this had disappeared as clover contents rapidly converged. Frequency of grazing had its greatest effect on clover content in year 1, with infrequent grazing resulting (P<0.05) in the greatest clover contents. Grazing intensity was an important determinant of clover content in years 2 and 3, where hard grazing resulted (P<0.05) in higher clover content. Digestibility of the herbage on offer ranged from 65 to 80%, and crude protein concentrations varied from 12 to 26%. In general, frequent grazing resulted in a digestibility of 2–4 percentage units higher than infrequent grazing, with hard grazing also tending to increase digestibility. Hard grazed treatments always had high crude protein concentrations in the herbage present before grazing, and there was a slightly higher concentration in frequently grazed herbage compared with herbage that was grazed less frequently. The white clover–perennial ryegrass swards generally responded best to a combination of frequent and hard grazing. However, neither white clover nor perennial ryegrass appears to be well adapted to the combination of soils, climate, irrigation and grazing by dairy cows that occurs in the northern irrigation region of Victoria, as evidenced by a rapid influx of weeds and the general decline in productivity over the duration of the experiment.