Optimizing productive and reproductive performance in the grazing cow

Abstract

The efficiency of milk production in pasture-based systems is heavily influenced by calving pattern, necessitating excellent reproductive performance in a short breeding season. Where grazed pasture is the major component of the diet, cows are underfed relative to their intake potential. A number of studies have identified body condition score measurements that are related to likelihood of both submission and conception. In general, fertility variables have favourable associations with circulating concentrations of glucose, insulin and insulin-like growth factor-1 and unfavourable associations with non-esterified fatty acids, beta-hydroxybutyrate and endogenous growth hormone. Nutritional strategies to impact these metabolic indicators have been utilised, but effects on herd fertility are inconsistent. Long term selection for improved genetic merit for fertility traits increases the likelihood of pregnancy establishment. This is achieved through more favourable metabolic status, advancing postpartum resumption of cyclicity, improved postpartum uterine health, stronger behavioural oestrous expression and increased luteal phase circulating progesterone concentrations. Use of synchronisation to maximise submission rates can advance the timing of pregnancy establishment and improve calving pattern. Incorporation of sexed semen into the breeding programme can increase the number of female calves born, but at the expense of reduced conception rates. Nevertheless, careful selection of the heifers and cows to be inseminated with sexed semen will minimize the reduction in fertility performance. This review outlines the unique challenges of pasture-based systems, and the roles of nutritional management, genetic improvement of fertility traits, synchronisation and incorporation of sexed semen in the breeding programme in regulating reproductive performance.