Abstract:
Proc. Aust. Soc. Anim. Prod. Vol. 19 GROWTH OF LAMBS OVER THE SUMMER ON FORAGE RAPE D. L. HOPKINSAB, K. L. PIRLOTA, A. H. K. ROBERTSA and D. E. JOHNSON 'Dept of Primary Industry, Tasmania, P.O. Box 180, Kings Meadows, Tas. 7249. BPresent Address: NSW Agriculture, P.O. Box 242 Cowra, N.S.W. 2794. A Supplying finished heavy lambs from pasture over summer in Tasmania is considered difficult. Previous work with spring-born lambs has shown a marked drop in growth rate over the late summer/autumn period (Hopkins et al. 1990). Fitzgerald (1985) reported that lambs grazing rape (Brassica napus) can achieve faster growth rates than those grazing pasture over the summer autumn period. With these facts in mind a study to investigate the value of forage rape as an alternative to pasture was conducted and the results are reported here. In October 1990, 3 ha were sown with Rangi rape and the area fertilised with 125 kg/ha super-phosphate and 150 g/ha of molybdenum. Herbage yield of the crop was estimated prior to grazing. Quadrat samples of green leaf and petiole, but not stem or senescing leaves, were harvested and oven dried to assess available dry matter. The 155 male lambs used were born in July-August to mixed-age Merino and Polwarth ewes. They were sired by either Suffolk or Poll Dorset rams. At marking all male lambs were made short scrotum. The lambs grazed the rape from 2-30 January 1991 when 54 weighed more than 44 kg liveweight, the criterion used for slaughter. These lambs were returned to the crop until 12 February. The remaining lambs grazed regrowth pasture. Available herbage of rape prior to grazing was 4180 kg DM/ha and the lambs were stocked at over 50 per/ha. The lambs had a mean (& s.d.) liveweight of 36.4 + 3.95 kg at the commencement of grazing and grew on average at 171 + 78 g/day for the first 28 days. Those lambs selected for slaughter grew at 161 & 43 g/day for the 41 days they grazed the rape and had a mean final liveweight of 47.7 + 2.29 kg. Their mean hot carcass weight was 22.4 + 1.41 kg (range 19.4-25.6) with a mean GR measurement of 12.5 & 2.34 mm (range 7-19). Forage rape has been shown to be a useful feed substitute over the summer for finishing lambs although individual animal performance was , variable as found elsewhere (Milne 1990). The combination of short scrotum lambs and forage rape resulted in 56% of the carcasses from the 54 lambs selected for slaughter conforming to the Elite lamb specification (2 22 kg, fat score 2 or 3). Of those carcasses outside the specification 4 were over-fat and 20 under weight. Choice of a heavier liveweight as the criterion for slaughter would have reduced the number of carcasses outside the specification but also the number turned-off. Reducing the stocking rate on the rape would allow more animals to attain a desirable liveweight for slaughter while on the crop. This work was partly funded by the Meat Research Corporation. FITZGERALD, J. J. (1985). Zr. J. Agric. Res. 24: 18 l-93. HOPKINS, D. L., GILBERT, K. D. and SAUNDERS, K. L. (1990). Amt. J. Exp. Agric. 30: 165-70. MILNE, J. A. (1990). ` Milk and meat from forage crops.' (British Grassland Society: Berkshire.) 254