Studies of Holstein-Friesian cattle bred for heavy or light mature live weight : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Institute of Veterinary, Animal and Biomedical Sciences, Massey University

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The new Animal Evaluation Model predicts that heavier live weight (LW) of the lactating cow reduces the profitability of the pasture-based dairying farm in New Zealand, because its effects on increased maintenance requirements are not fully compensated by the extra income generated from selling heavier culled cattle and surplus progeny. The work outlined in this thesis was intended to validate the expected effects of selection for differences in LW on actual LW from birth to maturity and on herbage intake and feed conversion efficiency (FCE) of growing cattle and lactating cows. It also investigated the existence of any associated effects on calving difficulty, calf mortality, onset of puberty and reproductive performance of the two lines of Holstein-Friesian (HF) cattle bred for heavy or light mature LW. These two lines have been developed at the Dairy Cattle Research Unit, Massey University, New Zealand, since 1989. The high genetic merit HF cows from the base herd have been mated to high genetic merit HF sires with either high or low breeding value (BV) for LW but with similar breeding worth (BW) in order to generate the heavy (H) and the light (L) mature LW selection lines. During the period 1994 to 1997, a series of experiments with growing heifers and lactating cows from the H and L lines, and analysis of data collected from the cows were undertaken to compare the two genetic lines. The BV's for live weight of the sires were 86 kg for the H and 31 kg for the L cows and the actual H animals were heavier at birth (41 vs. 35 kg) and at maturity (510 vs. 460 kg). In addition the BV's for milk (1037 vs. 737 l), milkfat (33.0 vs. 27.5 kg) and milk protein (31 kg vs. 22 kg) of H sires were also higher and the H cows produced significantly more milk (4708 vs. 4323 l/lactation), more milkfat (207 vs. 198 kg/lactation) and more milk protein (157 vs. 150 kg/lactation) than the L cows. However, the L sires had slightly higher breeding worth ($ 46 vs. $ 37) than the H sires and theoretically calculated and experimentally measured feed intakes and the resultant feed conversion efficiencies, confirmed that the L cows had slightly higher values for FCE than the H cows in three short-term grazing experiments and when FCE was calculated over multiple lactations. Therefore the basic assumptions in the Animal Evaluation Model seem to be correct. Sires of H cows had a higher proportion of USA Holstein genes in their pedigrees than the L sires. Consequently cows from the H line had a higher (≈27%) proportion of USA Holstein genes compared to cows from the L line (≈7%), whose sires were mainly of New Zealand ancestry. New Zealand bulls are progeny tested under grazing conditions and a very tight seasonal system of reproduction, whereas North American bulls are progeny tested under dairying systems of all year round milk production and feedlot feeding. There were significant differences in the pattern of grazing behaviour of H and L cows. The L cows displayed a more 'aggressive' pattern of grazing behaviour than H cows given by significantly longer grazing times (520 vs. 499 min/d), faster rates of biting (58 vs. 52 bites/min), higher number of total bites per day (31053 vs. 25046 bites/d), lower rumination times (471 vs. 572 min/d), and the selection of herbage of higher digestibility (72.0% vs. 69.3%). These results may reflect not only a difference in mature LW between the H and L cows, but may also reflect a strain of Holstein (i.e. NZ vs. USA Holstein) difference due to the sires' ancestry referred to above. There were no differences between H and L cows in the incidence of calving difficulty. However, offspring of bulls with high BV for rump width (i.e. wider pelvises) were more likely to face a difficult calving, and so were daughters of bulls with low BV for rump angle (i.e. less sloping pelvises). There were no differences between H and L cows for calf mortality. However, induced calves were more likely to die or undertake an emergency slaughter, and the H cows had significantly higher induction rates than the L cows (10.5 vs. 4.2%). The H heifers grew faster, ate more feed (4.3 vs. 3.8 kg/hd/d) and were heavier (241 vs. 221 kg) and older (325 vs. 300 d) at puberty than L heifers, and there were no differences between H and L heifers in pregnancy rate, age at first calving and first lactation yield of milk and milk components. There were only small differences in the reproductive performance of H and L cows after adjusting by differences in induction, calving date and percentage of USA Holstein genes in the cows. The L cows had slightly shorter intervals from first service to conception (13 vs. 17 d) and from the start of mating to conception (24 vs. 29 d), and slightly higher first service conception rate (65 vs. 54%), which translated in a more concentrated calving pattern and lower induction rate (4.2 vs. 10.5%) for the L cows. The results of this thesis indicate that selecting for heavier mature live weight produced the expected results of heavier animals with higher yields of milk and milk components, higher feed requirements and higher herbage intakes and slightly lower feed conversion efficiency than lighter mature live weight cows. However, there were also differences in grazing behaviour in which the L cows displayed a more competitive pattern of grazing behaviour than the H cows. The results of this thesis suggest that for the New Zealand seasonal system of milk production based almost completely on grazed pasture, lighter mature LW HF cows may have an advantage over heavier mature LW cows. Under the conditions of this experiments L cows were slightly more efficient, younger at puberty, had a more concentrated calving pattern, and were less prone to be induced to calve than heavier mature live weight HF cows.
Dairy cattle growth, Dairy cows, Milk production, Cattle reproduction