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Rovira P.,Instituto Nacional Of Investigacion Agropecuaria Inia Uruguay
Livestock Research for Rural Development

Lamb fattening has been integrated to rice-pasture systems of production in lowlands to keep the livestock sector competitive. The objective of the present study was to evaluate the effect of stocking rate (SR) and supplementation (S) on lamb performance grazing improved pastures (legumes + ryegrass) after rice harvesting. Sixty castrated Romney Marsh lambs 9 months old with an initial liveweight (LW) and body condition score (BCS, scale 1 to 5) mean ± SD of 35.1±2.8 kg and 2.8±0.27 points were divided randomly into balanced groups of six animals and assigned to five treatments with two replicates: 1) 10 lambs/ha, 2) 16 lambs/ha, 3) 16 lambs + S, 4) 24 lambs/ha, 5) 24 lambs/ha + S. The supplement used was high moisture sorghum grain (HMS) daily delivered at 1.1% of liveweight (as-fed basis). Botanical composition at the beginning of the study was 53% rice stubble, 38% legumes and 9% ryegrass and other grasses; with an overall chemical composition of 13.4% crude protein and 52.2% acidic detergent fiber. In unsupplemented treatments, final liveweight (LW) decreased from 50.8 to 37.5 kg as SR increased from 10 to 24 lambs/ha as a result of a significant SR effect on average daily gain (ADG) (206, 151 and 40 g/a/day for 10, 16 and 24 lambs/ha; respectively). Maximum ADG was achieved at an estimated daily allowance of 1.2 kg DM/head/day. There was a quadratic effect of SR on LW gain per hectare which decreased from 158 to 62 kg/ha (16 and 24 lambs/ha, respectively). For supplemented treatments, HMS improved ADG when compared at 24 lambs/ha (40 and 122 g/a/day without and with supplement, respectively) achieving a feed efficiency of 4.6 kg of supplement per kg of added gain above the unsupplemented animals. In addition, supplementation improved average BCS from 3.4 to 4.1 at 24 lambs/ha but 8% of those animals still failed to meet minimum target BCS for slaughter (3.5) at the end of the experiment. Although poor-drained rice soils can be a predisposing factor for the occurrence of footrot, lambs did not show clinical symptoms of the disease probably due to the low mean minimum temperature (6.0°C). Stocking rates between 10 and 16 lambs/ha are recommended for fattening lambs grazing improved pastures in poor-drained lowland rice soils. At higher stocking rates supplementation and/or extended grazing periods would be required to improve animal performance and to meet industry requirements for slaughter. Source

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