Growth performance, antibody response, and mammary gland development in New Zealand dairy replacement bovine heifers fed low or high amounts of unpasteurized whole milk

This study evaluated the influence of feeding low and high preweaning allowances of unpasteurized whole milk (MA) on intake, selected blood metabolites, antibody response, mammary gland growth, and growth of New Zealand (NZ) dairy heifers to 7mo of age. At 10 ± 2d of age (study d0), group-housed (6/pen) heifer calves (Holstein-Friesian×Jersey) were allocated to low (4L whole milk -1calf -1d; n = 7 pens) or high (8L whole milk -1calf -1d; n = 7 pens) MA for the next 63d. Calves were gradually weaned between d63 ± 2 and 73 ± 2. Calves in each pen had ad-libitum access to clean water, pelleted calf starter, and chopped grass hay from d1 to 91 ± 2d. At 92 ± 2d, all calves were transferred to pasture, grazed in a mob, and their growth and selected blood metabolites were measured until d209. All animals were weighed weekly during the indoor period (to d91) and then at d105, 112, 128, 162, 184, and 209. Skeletal growth measurements and blood samples to analyze selected metabolites were collected at the start of the experiment, weaning, and then postweaning on d91, and d201. Specific antibodies against Leptospira and Clostridia were quantified in wk7, 13, and 27. Mammary glands were scanned using ultrasonography at the start of the experiment, weaning, and d201. Feeding high versus low amounts of MA increased the preweaning growth in heifer calves (P = 0.02) without negatively affecting postweaning Average Daily Gain (ADG) (P = 0.74). Compared with heifers fed low MA, high MA fed heifers had greater increase in antibodies against Leptospira and Clostridia by 13wk of age (P = 0.0007 and P = 0.06, respectively). By 27wk of age, the antibody response was the same in heifers offered low or high MA. There was no effect of MA on the total size of the mammary gland, measured by ultrasonography, at weaning and 7mo of age. However, the greater MA was associated with more mammary parenchyma (P = 0.01) and less mammary fat pad (P = 0.03) in back glands at 7mo of age compared with heifers fed lower MA. In conclusion, feeding a high versus a low amount of unpasteurized whole milk increased the preweaning growth of New Zealand replacement heifers without negatively affecting their ADG during postweaning under grazing conditions. Feeding more (8 vs. 4L/d) unpasteurized whole milk positively affected antibody responses early in life and mammary gland composition by 7mo of age in dairy heifers reared for pasture-based dairy systems.