Abstract:
Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 EFFECT OF PERENNIAL GRASS NUMBERS ON STEER GROWTH K.D. GREATHEAD and J.N. BOULTWOOD Dept of Agriculture, Albany Highway, Abany, W.A. 6330 Perennial grasses combined with annual subterranean clover can improve animal production over annual-only swards in the high rainfall areas of Western Australia (WA) (Nicholas 1979). Cattle production from 4 perennial grass species (Currie cocksfoot, Brumby ryegrass, Triumph fescue and Sirosa phalaris) each growing in swards with annuals is being compared with annual-only swards in 3 replicates at each of 2 sites in the 700 mm rainfall zone in the south west of WA. These treatments are stocked with 2 weaner steers/ha in December each year and continuously grazed for 12 months. By December 1992, 15 of the pastures (6 annual and 9 perennial) had been grazed for at least 3 years while the others (15 perennial) had been sown in May and stocked from August 1992. This study investigated the effect of perennial plant density in early autumn on the average liveweight gain/hd in each plot from December 1992 to mid-April (range -22 to 51 kg), to mid July (range 33 to 148 kg) and to early October 1993 (range 120 to 243 kg). Plant counts were recorded in late March 1993 and ranged from 0-64/m2. At this time of year perennial grass plants can be readily identified among the low levels of annual pasture residue following early autumn rains. These numbers reflect summer survival and are likely to be maintained through the subsequent growing season. Table 1. Regression equations showing the effect of perennial grass plant counts (C plants/m2) at the end of March 1993, on steer growth (kg) from December 1992 to the times shown in 1993 The regression equations indicate significant effects of the perennial plant counts on animal growth to mid-April (P < 0.001) and to mid-July (P < 0.01). By October perennial grass counts recorded in March did not significantly affect liveweight gain. In all regression equations the value of the intercept was mainly determined by the time of year, with values increasing from less than 0 kg by mid-April to greater than 170 kg by October. Inclusion of the quadratic coefficient only significantly improved the R' value and reduced the RSD of the relationship of weight gains to April. The lower RSD of gains to April (10.7 kg) indicates a much closer fit than those to July ( RSD 25 kg). In 1993 the quadratic fit for gains to April showed no liveweight benefit when perennial plant counts were less than about 20/m? For benefits to be significant counts of closer to 40 plants/m' were necessary. As the summer of 1992/93 was drier than average, the benefit of perennial plants during summer/autumn can be attributed to their higher digestibility and their earlier production of accessibile green material than the annuals in response to autumn rains. The decline in effect of perennial grasses in mixed swards on weight gain as the season progresses is probably associated with compensatory growth of steers grazing the increasing quantities of high quality annuals that germinated in autumn. NICHOLAS, D.A. (1979). J. Agric. West. Amt. 20: 57-9. 413