Regrowth characteristics of lucerne under different systems of grazing management

Abstract

The regrowth characteristics of lucerne (Medicago sativa cv. Hunter River) were measured within an experiment conducted at Lawes, south-eastern Queensland, from 1971 to 1977 (Leach 1979). The experiment included six grazing and three mowing treatments: (1) R32/G4, (2) R32/G16, (3) R44/G4, (4) R44/G16, (5) R56/G4, (6) R40/G8, (7) R36/M, (8) R48/M and (9) R60/M, where R represents the days of rest between grazings (G) or mowings (M) and G the days of grazing after each rest period. These treatments formed cycle lengths of 36, 48 or 60 days, and regrowth experiments were usually undertaken when the ends of two or more defoliation cycles coincided. There were 15 regrowth experiments: experiments 1-9 were conducted over the first 720 days, experiments 10–14 over the next 720 days, and experiment 15 after another 720 days. In experiments 1-9 an initial harvest was made on day 0, and the regrowth was measured after 8 and 24 days. In experiments 10-14 there were harvests on days 0 and 24, and in experiment 15 on days 0 and 14. Amounts of regrowth were more strongly influenced by season and by soil moisture status than by imposed treatments, with final yields differing by up to 10-fold between winter and summer experiments. Seasonal differences in shoot size, and to a lesser extent in shoot numbers, were also much larger than treatment effects within experiments. These results appeared to be a consequence of the grazing and mowing systems being selected from within a recommended range for lucerne, and also of ensuring that moisture equivalent to the long-term average was received each month. Within experiments the yield of regrowth increased as the resting interval between defoliations increased, with a twofold difference between treatment extremes in some experiments. In general, grazing for 16 days led to better regrowth yields than grazing for 4 days which was in turn superior to mowing. Retaining residues after grazing generally increased regrowth, particularly following 16 days of grazing. However, residues greater than 80–100 g m-2 decreased regrowth. Most treatment differences were particularly evident on day 8, which indicated that treatments affected regrowth mainly through the speed of regrowth resumption after grazing or mowing. Although treatment effects on total regrowth yield were attributable to change in both shoot numbers and shoot size, the latter component was usually the more important determinant. Regrowth yield per unit area was maintained by compensating increases in yield per plant, whilst lucerne density fell from an initial 50 plants m-2 to about 30 plants m-2 over the first 3 years, but subsequent losses of lucerne plants led to decreased yields per unit area.