Abstract
Our objective was to investigate the effects of post weaning forage grazing on heifer development and fertility. Weaned (7.5 mo old) crossbred heifers (n = 139) were strip-grazed on alfalfa (A) or grass (G) pastures for 56 d with 2 replicates per treatment. Heifers were weighed every 2 wk and forage samples were collected on wk 1, 4, and 8. After grazing, all heifers were managed similarly. At 10 d before estrous synchronization, heifers weighed 381.7 ± 2.9 kg, and reproductive tract score (RTS) and antral follicle count (AFC) were determined by ultrasonography. Heifers were synchronized using the 14 d CIDR Split-Time AI protocol. Serum progesterone concentrations were determined at ultrasonography and synchronization. Heifers were considered pubertal when progesterone concentration was >1 ng/mL. A mixed model was used to determine effects of forage, wk, and their interaction on diet composition, and forage effects on heifer bodyweight at the end of treatment (BWT) and at ultrasonography (BWU), average daily gain (ADG), RTS and AFC. Correlations between variables were also tested. Crude protein was affected by forage and wk (P < 0.05), and averaged 15.5% of dry matter for A and 8.87% for G. There was a forage by wk interaction on acid detergent fiber (P < 0.02). Forage did not affect BWT or BWU, but A heifers gained more than G heifers (0.41 ± 0.03 vs. 0.20 ± 0.03 kg/d; P = 0.03). Both BWT and BWU were correlated with RTS, regardless of treatment (r = 0.40 and 0.40, respectively, P < 0.01). There was no effect of forage type on pubertal status (P= 0.85) or pregnancy rate (P = 0.67). In summary, replacement heifers were grazed on alfalfa or grass pastures after weaning, which resulted in different growth rates but did not compromise fertility measures.
Keywords: grazing, heifer, reproduction