The productivity of irrigated legumes in northern Victoria. 2. Effect of grazing management

The productivity of irrigated white (Trifolium repens L.) and red (Trifolium pratense L.) clover swards was compared in an experiment of more than 3 years duration. It was hypothesised that white clover would be more productive than red clover when defoliation was frequent and intense, and less productive when defoliation was infrequent and lax. The experiment was a factorial design involving 2 species of clover [white clover (cv. Haifa) and red clover (cv. Redquin)], 2 grazing frequencies and 2 grazing intensities (with the criteria for both being based on quantities of herbage present before/after grazing). There were 4 extra treatments sown: perennial ryegrass (Lolium perenne L. cv. Grasslands Nui) and white clover (cv. Haifa), lucerne (Medicago sativa L. cv. Validor), Persian clover (Trifolium resupinatum L. cv. Maral) or subterranean clover (Trifolium subterraneum L. cv. Trikkala), but only 1 defoliation treatment was used for each of these treatemnts. There were 4 replicated blocks of all treatments. Apparent growth rates [calculated from measurements of dry matter (DM) removed by grazing] of white clover ranged from a low of 10 kg DM/ha.day in winter to a high of 70 kg DM/ha.day in summer. The growth rates of white clover swards were superior to those of ryegrass and white clover swards over summer, but were generally lower from May to October. In 2 of the 4 years, frequent grazing of white clover resulted in greater (P<0.05) production than infrequent grazing (average of 12.8 v. 10.7 t DM/ha) whereas intensity of grazing only affected DM net accumulation in the first year (P<0.05). The data show no evidence of a decline in productivity over time. Sward structure of white clover was influenced by grazing treatment with the numerically highest yielding treatment (frequent and hard) having the highest density of stolon tips (vegetative buds). In relation to days of regrowth, the frequently grazed treatment had higher levels of net photosynthesis in spring and summer compared with the infrequently grazed treatment. The frequently grazed treatment achieved positive carbon balance immediately after grazing and reached maximum levels of photosynthesis at 8–10 days, whereas the infrequent treatment showed negative carbon balance for the first 2–3 days after grazing with maximum photosynthesis being achieved later than in the frequently grazed treatment. When net photosynthesis was related to leaf area, there were fewer differences between the 2 treatments. The exception was in spring when photosynthesis was lowest where the initial leaf area was highest in the infrequent and hard treatment. Maximum photosynthesis was achieved at diminishing leaf area index from spring through to winter. Red clover was the most productive legume in the first year after establishment, but it did not persist beyond the second year and its DM net accumulation was reduced by more frequent grazing (12.4 v. 15.3 t DM/ha in the first year and 6.1 v. 9.1 t DM/ha in the second year; P<0.05). The DM net accumulation of lucerne was greater than that in any other treatment (an average of 16.7 t DM/ha in the 2 completed years), whereas the annual legumes, subterranean clover and Persian clover, averaged 6.6 and 10.7 t DM/ha.year, respectively. The seasonal growth rate data showed that lucerne had very good summer production whereas the annuals tended to be at least as good as the perennials from May to October.