Crecimiento predestete de corderos en sistemas intensivos de pastoreo y manejo reproductivo en el altiplano central de México * Preweaning growth in lambs under intensive grazing and reproductive management systems in central Mexico

To study factors affecting birth weight (BW), weight at 30 days and 60 d, data from 917 Columbia lambs were analyzed. Lambs were born in april (N-B) from yearly breeding, or in april-may (A-M), december-january (DJ) and august-september (A-S) from an every eight months breeding program (I-B). Ewes and lambs were grazed on irrigated grass-legume temperate pastures; lambs had access to creep feeding from 15 d of age. Fixed effects in the variance analysis were: ewe age; single or twin birth; sex and year or season of birth. Simple correlations were obtained between BW and weights at 30d and 60d and of these with ewe body condition. The BW (5.7 ± 0.06 kg) and weight at 30 d (15.0 ± 0.17 kg) in I-B were higher (P 0.05), but were at 30d (14.7 ± 0.22 kg vs. 13.2 ± 0.18 kg). I-B lambs born in D-J were always heavier (P < 0.05): 6.2 ± 0.08 kg, 5.3 ± 0.10 kg and 5.5 ± 0.09 kg; 16.2 ± 0.22 kg, 14.8 ± 0.27 kg and 13.8 ± 0.25 kg; 22.2 ± 0.33 kg, 21.3 ± 0.39 kg and 20.8 ± 0.36 kg for BW, 30 d and 60 d in D-J, A-M and A-S, respectively. Correlations between weights were always positive (P < 0.05), and only in N-B was there negative correlation between ewe body condition and weight (P < 0.05).