Nutritional, genetic and meat quality aspects of once-bred heifer beef production systems in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University
Issues related to the nutrition, management, carcass and meat quality traits, and the profitability of "Once-Bred Heifer" (OBH) beef production systems under New Zealand pastoral conditions were investigated in this research programme. Once-bred heifer beef production systems involve surplus heifers from the dairy industry which are mated to beef sire breeds at about 15 months of age and rear their calves for 3-6 months before being slaughtered at 30-32 months of age. Both the heifer dams and their progeny are thus prime meat-producing animals. The performance of once-bred v. unbred heifers and of early-weaned (EW) heifers (heifers weaned at day 84 of lactation "L84") v. normal-weaned (NW) heifers (heifers weaned at L147) were studied in the first two trials. In the third trial, Hereford x Friesian (H x F) v. Simmental x Friesian (S x F) heifers offered a restricted herbage allowance (RHA, an allowance to maintain weight) v. normal herbage allowance (NHA, an allowance to grow at 0.6-0.7 kg/d) during mid pregnancy (from pregnancy day 114, "P114", to P214) were studied in an attempt to investigate the effects of dam genotype and prior herbage allowance on the performance of heifer dams (growth rate, reproduction and carcass and meat quality traits) and their progeny (growth rate and weaning weights). A gross margin analysis was also performed to evaluate the profitability of alternative OBH beef production systems. Once-bred v. unbred Hereford x Friesian heifers consumed similar amounts of herbage organic matter (OM) during the period equivalent to late pregnancy of the former group (4.72 v. 5.15 kg OM/hd/d), but lactating heifers consumed more herbage OM than the comparable unbred group (11.36 v. 9.19 kg OM/hd/d, P < 0.05) to support the growth of their calves, as well as themselves. Unbred heifers had greater carcass weights and higher dressing-out percentages than once-bred heifers. However, the differences in other carcass and meat quality traits between the heifer groups were small, indicating that once-bred heifers are capable of producing meat comparable in quality to that of unbred and empty heifers. Higher gross margins ($5-10/Stock Unit (SU)) were calculated for once-bred v. unbred heifers. During the 10 days immediately prior to weaning, NW heifers had organic matter, dry matter and energy intakes which were slightly higher (P < 0.10) than those of EW heifers (weaned at L84). Weaning caused a slight weight loss in both early- and normal-weaned heifers for the first 30-45 days post-weaning, but liveweight (LW) was recovered after this period. During L84 - L147, EW heifers had a higher daily liveweight gain (LWG) than NW heifers (0.79 v. 0.51 kg/d, P < 0.01). Calves weaned at L84 had significantly lower daily LWG than NW calves during the period L84 - L147 (0.73 v. 1.30 kg/d, P < 0.001). This resulted in EW calves being 30 kg lighter (P < 0.001) than NW calves at the time of weaning for the latter group. Early-weaned heifers reached the target slaughter weight in March and, overall, had slightly better carcass and meat quality characteristics than those of the normal weaned group. Although similar gross margins were calculated for EW ($41.68/SU) v. NW ($42.00/SU) heifers, early weaning offers advantages to OBH beef production systems through increased flexibility of grazing management and selling times for animals. From P114 until P214, NHA heifers had significantly (P < 0.001) higher growth rates (0.72 ± 0-03 kg/d) than the RHA group (0.16 ± 0.02 kg/d). This resulted in a higher LW in NHA heifers at P214 (P < 0.001) and P270 (P < 0.01) than the RHA heifers. Gestation length, calving score, LW loss at calving and calf birth weight were not affected by dam genotype (Hereford x Friesian v. Simmental x Friesian) or prior herbage allowance. Meat quality traits were not affected by dam genotype, but it was concluded that the use of Simmental x dairy heifers in a OBH beef production system increases carcass weights of heifer dams in comparison to those of heifers derived from traditional British beef x dairy animals. However, gross margins were similar for H x F and S x F heifer groups indicating that little incentive would exist for dairy farmers to use sires of the large European breeds (e.g. Simmental) rather than Hereford sires which are commonly used to mate first-calving heifers in the dairy industry. The results are discussed in the context of the development of once-bred heifer beef production systems in New Zealand.