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-Liveweight gain was assessed in lambs grazing perennial ryegrass (Lolium perenne) bred for an elevated water-soluble carbohydrate (WSC) concentration (experimental var. Ba11353) compared to an intermediate WSC control (var. AberElan). Both grass varieties had similar heading dates and overall forage yield characteristics. The two grasses were sown separately in each of three replicate sward plots (0.137 ha each) laid out in a randomised block design. A core group of five Welsh Mountain ewes and their singleton lambs (Bluefaced Leicester, sire), initial liveweights of 50 and 14 kg (s.e. 0.6 and 0.3), respectively, were continuously stocked on these plots over three consecutive 21 d experimental periods. Swards were maintained at a target surface height of 6-7 cm with the use of sward adjustment animals. Core sheep were weighed every three weeks and adjustment animals whenever they were added to or removed from the plots. Lamb liveweight gain (g . d -1 ) was calculated from the core group of lambs. Lamb production (kg liveweight . ha -1 over each 21d period) was calculated from the liveweight change of all the lambs. Water-soluble carbohydrate concentrations were higher (P = 0.03) and neutral detergent fibre concentrations (NDF) were lower (P = 0.01) in the high WSC grass (HWSC) compared to the control. In vitro dry matter digestibilities (DMD) were higher for HWSC (P < 0.01). Lamb liveweight gain (P = 0.03) and production (P = 0.06) were greater on HWSC compared with the control by approximately 12 and 23%, respectively. The increased lamb production resulted from the combined effect of higher liveweight gains and carrying capacity on HWSC. Correlation matrices indicated strong relationships between liveweight gain and grass WSC concentration (r = 0.67; P < 0.05) and fibre concentration (r = -0.73; P < 0.05). The results show that grazing forages selected for elevated concentrations of WSC and reduced fibre, compared to a control increases the liveweight gain of suckling lambs.water-soluble carbohydrate / grazing / lambs / Lolium perenne / liveweight gain M.R.F. Lee et al. 442

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