Abstract:
Cattle farmers in Ghana want larger animals with better milk yield and have therefore focused on improving the indigenous tropical Sanga breed by crossing with the temperate Friesian breed. However, in the Friesian-Sanga crossbred, a major limitation to reproductive performance is extended postpartum anoestrus, perhaps exacerbated by under-nutrition under field conditions. We therefore tested whether the breeds vary in the circulating concentrations of metabolic hormones that mediate the effects of nutrition on the reproductive axis during the postpartum period. Two herds comprising 16 Sanga, and 16 Friesian-Sanga cows, were grazed separately on natural pastures of similar nutritive value with no feed supplements. Every week during weeks 1 to 10 postpartum, the cows were weighed, body condition score (BCS) was assessed, and blood was sampled. Resumption of ovarian activity, detected from progesterone profiles, was used to classify cows as 'early-cycling' (≤45 days), 'late-cycling' (46-70 days) or 'non-cycling' (after 70 days). Resumption of ovarian cyclicity, body weight and body condition score did not differ between breeds, but daily milk yield was lower for Sanga (1.01±0.02. kg/day) than for Friesian-Sanga crossbreds (1.57±0.02. kg/day; P<0.001). Compared with Friesian-Sanga crossbreds, Sanga cows had greater plasma concentrations of IGF-I (18.7±0.7 versus 14.8±0.7. ng/mL; P<0.001) and leptin (1.12±0.01 versus 0.96±0.01. ng/mL; P<0.001), but the two breeds had similar concentrations of GH and insulin. After calving, the IGF-I/GH ratio recovered more rapidly in the Sanga than in the crossbred (. P=0.01). In both breeds, early-cycling cows had greater concentrations of IGF-I and leptin than late- or non-cycling cows. It was concluded that, during the postpartum period, the Sanga breed is better able to avoid the effects of negative energy balance, maintain metabolic homeostasis and prevent uncoupling of the GH-IGF axis, compared with the Friesian-Sanga crossbreed. © 2014 Elsevier B.V.