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

Lay Summary This study evaluated the effect of unpasteurized whole milk allowance on intake, antibody response, mammary gland growth, and growth performance of heifers until 7 mo of age. Feeding greater (8 L center dot d(-1)) vs. lower (4 L center dot d(-1)) milk allowance to heifer calves increased preweaning body weight without having any negative effect on postweaning growth under grazing. Heifers fed high milk allowance had significantly better antibody responses against Leptospira and Clostridia by 3 mo of age and had more mammary parenchyma (potential milk making tissue), and less mammary fat pad (supporting tissue) by 7 mo of age. Greater milk feeding of calves resulted in greater growth rates to weaning, no negative effects on growth on pasture, at 13 wk of age had significantly greater serum-levels of antibodies against Leptospira and Clostridia, and an increase in potential milk making tissue in their mammary glands at 7 mo of age. 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 7 mo of age. At 10 +/- 2 d of age (study day 0), group-housed (six center dot pen(-1)) heifer calves (Holstein-Friesian x Jersey) were allocated to low (4 L whole milk center dot calf(-1)center dot d(-1); n = 7 pens) or high (8 L whole milk center dot calf(-1)center dot d(-1); n = 7 pens) MA for the next 63 d. Calves were gradually weaned between days 63 +/- 2 and 73 +/- 2. Calves in each pen had ad-libitum access to clean water, pelleted calf starter, and chopped grass hay from day 1 to 91 +/- 2 d. At 92 +/- 2 d, all calves were transferred to pasture, grazed in a mob, and their growth and selected blood metabolites were measured until day 209. All animals were weighed weekly during the indoor period (to day 91) and then at days 105, 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 day 91, and day 201. Specific antibodies against Leptospira and Clostridia were quantified in weeks 7, 13, and 27. Mammary glands were scanned using ultrasonography at the start of the experiment, weaning, and day 201. Feeding high vs. 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 with low MA, high MA fed heifers had a greater increase in antibodies against Leptospira and Clostridia by 13 wk of age (P = 0.0007 and P = 0.06, respectively). By 27 wk 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 7 mo 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 7 mo of age compared with heifers fed lower MA. In conclusion, feeding a high vs. 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. 4 L center dot d(-1)) unpasteurized whole milk positively affected antibody responses early in life and mammary gland composition by 7 mo of age in dairy heifers reared for pasture-based dairy systems.