Growth analysis in light lambs raised under different management systems

A quadratic polynomial model with two random regression coefficients was used to describe the growth pattern of 130 single male lambs from the Churra Tensina breed which were raised under four different management systems. Treatments were: lambs in drylot with dams indoors (DR-IND), lambs in drylot with rationed grazing ewes (DR), lambs supplemented in creep feeders continuously with dams on pasture (GR + S) and lambs with dams continuously on pasture (GR). Lambs from DR-IND and DR were weaned at 49.4 ± 5.0 days old. All lambs were weighed at birth and thereafter at weekly intervals until slaughter at the target live-weight of 22–24 kg (23.3 ± 1.5 kg).The model fitted accurately weights (R2 = 0.96). The effect of treatment on the linear coefficient was not significant (average 267 g/day; P > 0.05). However, the quadratic coefficient varied across management systems (0.027 ± 0.12, 0.52 ± 0.17 and 0.22 ± 0.12 g/day2 in DR-IND, DR and GR + S, respectively, vs. −0.26 ± 0.12 g/day2 in GR; P < 0.001). Although standard errors increased at higher ages, the residual variance was 43% reduced when using two random regression coefficients. Around 60% of the variation in the data set was accounted for by allowing the intercept and the slope to vary across individuals.It was concluded that the model was suitable to describe a nearly linear growth pattern in light lambs and accounted for mean live-weight adjustment within lots resulting from removal of lambs that achieved the slaughter live-weight earlier.