Analysis of lactation length variability and its relationship to cow milk production

Lactation length is a key trait closely associated with both productivity levels and reproductive characteristics in dairy cattle. This study aimed to analyse the factors influencing lactation length variability and its impact on lactation curve formation, as well as the quantitative and qualitative traits of dairy herd productivity. The study utilised data from 604 complete lactations of cows that calved at PJSC “Plemzavod “Stepnoi” (Zaporizhzhia Region, Ukraine) between 2014 and 2017. Among the studied cows, lactation length ranged from 173 to 1,150 days, with a mean value of 356.1 ± 4.1 days. Cow age (in lactations) had a significant effect on lactation length (P < 0.001). No significant effect of the bull sire was observed on the lactation length of primiparous cows. However, when all lactations were considered together, a significant bull sire effect (P = 0.005) on lactation length was identified. Additionally, lactation length variability in the studied group was influenced by the year and season of calving. Overall, the lowest LSMestimated lactation length was observed in multiparous cows that calved in the autumn months (332.9 days), while the highest values were recorded in cows that calved in winter or autumn (ranging from 386.3 to 397.0 days). Lactation length was significantly associated with total milk yield (P < 0.001), whereas a significant correlation with 305-day milk yield was established only in multiparous cows (P = 0.036). The fat and protein milk percentages of the studied cows were also significant (P ≤ 0.001-0.020) but negatively associated with lactation length. When approximating the lactation curves of the studied cows using Wood’s model, it was found that as lactation length increased, the estimated value of coefficient “a” in Wood’s model also increased, whereas the estimated values of coefficients “b” and “c” decreased. Overall, longer lactation length was associated with an increase in daily milk yield estimates for months 8-10, contributing to improved lactation curve persistency