Modelling milk yield and calf performance of beef suckler cows on pasture-based systems

Abstract Text: Milk production of beef suckler cows is the main factor determining live weight of calves at weaning. However, the milk yield of beef suckler cows in Ireland is declining due to a reduction in the proportion of replacements sourced from the dairy herd and an emphasis on breeding for terminal traits. Correspondingly, calf weaning weights are also declining. Additionally, in suckler beef systems where grass, both grazed and conserved, forms a major part of the feeding system, beef suckler cows have to cope with seasonal changes in feed resource availability over the annual production cycle. The aim of current study was to develop a dynamic model to evaluate how the dynamics of milk production effects on the growth performance of beef suckler calves, within a pasture-based production system. A dynamic Grange Suckler Cow-Calf Model (dGSCCM) was developed that simulates energy partitioning, milk production and calf performance of two contrasting beef suckler cow genotypes; Charolais (C) and Charolais x Holstein-Friesian (CF). The parameter coefficients and equations used in the model were derived from published literature and established databases. The performance of cows and calves were determined by the genetic potential, physiological status, and availability of feed. The milk production profile of both beef suckler cow genotypes were generated through Irish data fitted to the lactation curve of Wood. The average daily milk yield for first and second parity was 7.35 and 8.70 kg for C and 11.12, and 13.10 kg for CF, respectively. Corresponding weaning weights at 240 days of age were 282 kg and 301 kg for C and, 332 kg and 355 kg for CF. Sensitivity analysis was carried out by changing milk yield ±20%. It showed that calf weaning weight is highly sensitive to cow milk production; each additional kg in daily milk yield increased weaning weight by 10.2 kg for CF in second parity to 14.3 kg for C in first parity. The model provides a basis for the evaluation of the growth response of beef suckler calves based on dynamics of milk production of their dams.

Keywords: Calf live weight, dynamic modelling, lactation curve, pasture based systems