Effects of tissue mobilisation from lactating sheep on the growth of lambs

Tissue mobilisation by ewes during lactation and its association with suckling lamb growth was examined. The study was conducted at the Lincoln University sheep unit from September to December 2015 using ewes that all gave birth to lambs but then reared with no lambs (n=9), one lamb (n= 25), two lambs (n=30) or three lambs (n=10). All ewes and lambs were set-stocked as one mob on ryegrass (Lolium perenne) and white clover (Trifolium repens) pastures under normal farming management. Nutrient status of the ewes was assessed at pre-lambing (day 1 of set-stocking), mid-lactation (day 63) and weaning (day 105) using live weight (LW), body condition score (BCS) and computed tomography (CT) measurements. Mobilisation of tissue reserves was determined by comparing the arithmetic means for LW, BCS and CT (fat, lean and bone tissue in the carcass) measurements at successive time intervals. The change in tissue reserves for ewes was correlated to the live weight gain (LWG) and weaning (WW) of lambs as a measure for assessing the lactation performance of ewes. Mobilisation of body reserves by ewes during lactation was influenced by number of lambs reared. Ewes that did not rear lambs did not mobilise body reserves during lactation, while ewes that reared lambs showed marked declines in LW (P = 0.024), BCS (P = 0.036), carcass fat (P = 0.019) and net energy (P = 0.03), but not for lean and bone tissue. At the end of lactation (weaning), LW, BCS, carcass fat and NE declined by at least 19% of pre-lambing measurements, which became more profound with increasing number of lambs reared. Twin and triplet-bearing ewes mobilised more energy reserves than those with single lambs during early lactation, while all lamb-rearing ewes showed similar tissue mobilisation rates during the late lactation period. Although the LWG of lambs was low, lambs reared as singles grew 1.8 times faster than those reared as twins and triplets (P < 0.001). Lamb LWG and WW were poorly associated with changes in ewe body reserves during lactation. Moreover, ewes that mobilised more energy reserves tended to have lambs with a lower WW. Taken together, the findings of this study do not support the hypothesis that greater mobilisation of tissue reserves in ewes during lactation is associated with superior LWG and WW of lambs