Efecto del grupo genético y la oferta de forraje sobre la respuesta productiva de vacas de cría en campo natural

The objective of this study was to evaluate the interaction between forage allowance (FA) (9 and 6 kg DM/ kg LW annual average for high (H) and low (L) FA respectively that vary seasonally) and crossbreeding (Hereford and Angus purebreds (P) and F1 crosses (C)) on forage mass, stocking rate, body condition score (BCS) and live weight (LW) of the cow, LW of the calf, ovarian activity (follicle size, presence of corpus luteum), and total pregnancy in a production cycle (start in autumn/second trimester of gestation at the end of summer/final weaning at 147 days pospartum (dpp)). Thirty-five pregnant multiparous cows were used in a randomized block design with a factorial arrangement of two FA treatments and the crossbreed. Suckling restriction (SR) and flushing (Fl) were applied at the beginning of calving (52 dpp). Forage mass, BCS and LW were determined monthly and FA was adjusted using the put and take method (Mott and Lucas, 1952). Ovarian activity was recorded before suckling restriction and flushing (Fl), during Fl and post Fl. Total pregnancy was evaluated by ultrasound at 147 dpp. H had higher mass forage than L (2064 and 1368 ± 234 kg DM/ha; p <0.05). Low precipitation levels would have restricted forage growth and resulted in lower levels of forage mass and stocking rate than expected mainly in L. H improved BCS during the fall (4.3 and 4.0 ± 0.12 kg DM/ha; p<0.05) and follicle size (11.8 and 8.16 ± 0.79 mm; p<0.01) relative to L. C cows had higher BCS from -131 to -94 dpp (p<0.05) and from -24 to 140 dpp (p<0.05), larger follicle size (10.77 vs 8.55 ± 0.77 mm; p=0.06) and tended to have higher final pregnancy than P cows (85 vs 48 ± 12%, p=0.1). Calves from P cows on L had less weight than the other treatments at weaning (163 vs. 173 ± 6.8 kg, p<0.05). H forage allowance positively affected BCS, follicle size and calf weight at weaning of P cows while C cows had higher BCS, follicle size, higher pregnancy and calf weight at weaning on L compared to P.