Influence of end-grazing forage residual and grazing management on lamb growth performance and crop yield from irrigated dual-purpose winter wheat

Abstract

The effects of end-grazing forage residual and continuous v. rotational grazing systems on prime lamb performance, grain yield and quality were examined in an irrigated dual-purpose winter wheat (cv. Mackellar) crop in Tasmania. The design was a two end-grazing residual (400 and 800 kg/ha of dry matter (DM) at Zadoks Growth Stage 30, Low and High respectively, 0.2 ha plots) × two grazing system (continuously, or rotationally grazed in four subplots) factorial, replicated three times. Mixed-sex, second-cross lambs [37 kg liveweight (LW), 2.5 body condition score, 45 kg DM/head initial feed allowance] grazed for a total of 46 days before removal. Initial feed availability was 1875 kg DM/ha, with final residuals of 520 ± 57 and 940 ± 70 kg DM/ha for the Low and High treatments respectively. Particularly for the Low residual, in vitro DM digestibility and crude protein at stem elongation were reduced (P < 0.05) by rotational compared with continuous grazing. The weekly lamb growth rate (g/day) during the first 5 weeks of grazing was linearly related to average weekly available DM in kg/ha (GR = 0.35 ± 0.041 × DM – 194 ± 49.0, P < 0.01, R2 = 0.56). Total LW produced (336 ± 11.7 kg/ha), and grain yield (6.9 ± 0.21 t/ha), protein (11.4%), screenings <2.2 mm (10.9%) and 100 grain weights (3.82 g DM) were not different between treatments. There were no advantages of rotational grazing compared with continuous grazing. Irrigated dual-purpose winter wheat can be continuously grazed by lambs up to a 500 kg DM/ha residual at stem elongation without compromising total LW produced, grain yields or grain quality.