Persistent effects of neonatal diarrhea on Holstein dairy cow lactation performance through multiple parities.

Calf diarrhea, one of the most common stresses causing by heifer rearing, affects most Chinese dairy herds, yet its long-term consequences on lactation performance remain poorly quantified. This study explored whether calf diarrhea has a long-term effect on the lactation performance and Dairy Herd Improvement-related indices of dairy cows and its influence during different lactation periods. This retrospective study included a total of 1,907 Holstein dairy cows from a large-scale farm in northern China (700 in the diarrhea group and 1,207 in the healthy group). A mixed effects model and mediation analysis were used to evaluate the effects of calf diarrhea on growth performance, milk yield, milk composition, and other lactation parameters at different parities and lactation periods. The results revealed that the negative effect of calf diarrhea on lactation performance was persistent and varied by parity. The 305-d milk yield and peak milk yield of cows at each parity in the diarrhea group were significantly lower than those in the healthy group (P < 0.05). Growth performance, such as withers height (median: 134.00 vs. 135.00 cm), was more significantly affected in the first parity (P < 0.001). With increasing parity, the difference in body condition scores narrowed, but the loss of milk production continued to accumulate. For the second and third parities, cows with calf diarrhea presented significant reductions in 305-d milk yield, with decreases of 287.36 kg and 1,086.76 kg in the median comparison, respectively (P < 0.05). The corrected milk yield and milk fat percentage during the early postpartum and early lactation periods decreased more significantly in multiparous cows, and the daily milk production in the middle and late lactation periods remained consistently lower than that in the healthy group (P < 0.05). Mediation analysis confirmed that milk production played a key mediating role in the pathway by which diarrhea affected the milk fat percentage. Our research revealed that calf diarrhea is a critical early-life health challenge, with consequences extending beyond the calf period to significantly affect lactation potential. Our findings underscore the need for early-life interventions to mitigate lactation losses in high-parity herds.