Positive relationships between body weight of dairy heifers and their first-lactation and accumulated three-parity lactation production

This study investigated the relationships between body weight (BW) and milk production of 140,113 New Zealand dairy heifers. Heifers were classified into 5 breed groups: Holstein-Friesian, Holstein-Friesian crossbred, Jersey, Jersey crossbred, and Holstein-Friesian-Jersey crossbred. Body weights were assessed at intervals of 3 mo from 3 to 21 mo of age and their relationships with first-lactation and accumulated milk production over the first 3 lactations (3-parity) were analyzed. We found positive curvilinear relationships between BW and milk production. The response to an increase in BW was greater for lighter heifers compared with heavier heifers, indicating possible benefits of preferentially feeding lighter heifers to attain heavier BW. Within the age range and BW range studied, an increase in BW was always associated with an increase in first-lactation energy-corrected milk (ECM) and milk solids (milk fat plus milk protein) yield for breed groups other than Holstein-Friesian. For Holstein-Friesian heifers, there was a positive relationship between BW and ECM and milk solids yields for all ages except for 3 mo of age, when no relationship existed. These results show the potential to increase first-lactation milk production of New Zealand dairy heifers by increasing heifer BW. Likewise, for 3-parity accumulated yields, the BW at which maximum ECM and milk solids yields occurred were at the heavier end of the BW range studied. The costs of rearing a heifer are incurred regardless of how long she remains in the herd. Potential bias exists from considering only cows that survived to lactate each year if particular cows had better survival than others. Therefore, the data in the current study for 3-parity production includes all heifers that were old enough to have completed 3 lactations, regardless of whether they did or not. Including the heifers that did not complete all 3 lactations describes the effect that BW of replacement heifers has on accumulated milk yields without discriminating whether the increased milk yield came from greater survival or from greater production per surviving cow. Further research on the relationships between BW and survival of heifers is required to confirm whether the heavier heifers survived longer than the lighter heifers, but could explain why the relationship between BW and 3-parity milk yields was more curvilinear than the relationship between BW and first-lactation milk production. Holstein-Friesian heifers that were 450 kg in BW at 21 mo of age were estimated to produce 168 and 509 kg more ECM than 425-kg Holstein-Friesian heifers in first-lactation and 3-parity accumulated yields, respectively. A further increase in BW at 21 mo of age, from 450 to 475 kg, was estimated to result in 157 and 409 kg more ECM in first-lactation and 3-parity accumulated yields, respectively. Consequently, for heifers that were average and below average in BW, considerable milk production benefits would occur over the first 3 lactations by improving rearing practices to result in heavier heifers throughout the precalving phase.