Pre- and post-weaning nutritional management of dairy heifer calves on lifetime milk yield performance and milk composition as dairy cows within New Zealand pasture-based systems : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Animal Science, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand

Abstract

Early-life nutrition and management practices of dairy replacement calves are recognised as factors influencing growth, development, and first lactation performance and milk composition as a heifer or cow. In New Zealand’s predominantly pasture-based dairy systems, there remains limited knowledge of how pre- and post-weaning nutritional and grazing management strategies affect lifetime milk yield, milk quality, and udder health. The present study was conducted to evaluate the effects of various pre- and post-weaning nutritional and grazing management strategies on lifetime milk production, milk composition, and somatic cell score (SCS) of dairy cows in a once-a-day milking system, while also considering breed and seasonal variations in lactation. The study was conducted using two cohorts of dairy heifer calves born in 2014 (n = 54) and 2015 (n = 50) in a New Zealand pasture-based dairy farm. The 2014-born calves received either a high milk allowance: 3L of milk fed twice daily for one week, followed by 4L per feed, twice daily until weaning; or a low milk allowance: 2 L of milk fed twice daily until weaning. After weaning the calves from each milk allowance were grazed on either perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) pasture or a forage crop of chicory (Cichorium intybus L.) and plantain (Plantago lanceolata L.). This created four treatment groups: High Milk–Pasture (HMPA), High Milk–Crop (HMCR), Low Milk–Pasture (LMPA), and Low Milk–Crop (LMCR). The 2015-born calves were managed under different pre-weaning grazing systems, receiving the same amount of milk, but were on pasture prior to weaning, either under rotational grazing (RG) or set stocking (SS), and a third group had a post-weaning grazing treatment of preferential grazing (PG), which had high-quality pasture supplemented with concentrate feed. Milk production (kg), milk composition yield (fat, protein, milksolids), and milk components percentages and somatic cell score (SCS) were measured across multiple lactations (2016/17 2023/24 for the 2014 -born calves and 2017/18–2023/24 for the 2015-born calves). The breed of the calves used for this study was Holstein-Friesian (HF), Jersey (JE), and Crossbred (XB, i.e., HF×JE). The 2014-born heifers receiving a high milk allowance and grazed on pasture post-weaning had greater lifetime milk yield (kg) (P < 0.05) compared with the low milk–forage crop treatments. For the 2015-born calves, preferential post-weaning grazing resulted in greater lifetime milk yield (kg) (P < 0.05) compared with pre-weaning, rotational, and set stocking grazing strategies, while milk component yields (kg) and composition percentages did not differ between treatments in both cohorts. In both cohorts, composition yield (kg) and percentages peaked during mid-lactation (4th to 6th lactation). Holstein-Friesians produced higher milk volumes, Jerseys higher milk fat concentrations, and crossbreds demonstrated intermediate milk yields but greater fat and protein composition, likely due to heterosis. When compared to literature, somatic cell scores (SCS) were moderately increased, likely due to the once-a-day milking system in this study but remained within healthy and acceptable ranges throughout the lactation seasons considered, indicating udder health was not impacted by early-life management. Overall, early-life nutritional and grazing management strategies of calves have a considerable influence on lifetime milk yield (kg) and a consistent effect on milk components when the calves’ become heifers and cows in New Zealand pasture-based dairy systems. Breed and lactation season were also shown to be stronger drivers for lactation performance.