Extended lactation in high-yielding Holstein cows: Characterization of milk yield and risk factors for lactations >450 days

• Cumulative milk yield of cows with lactation >450 days is about 70% that produced in 305 days.
• Extended lactations up to around 800 days are feasible in high-yielding Holstein cows.
• Reproductive and metabolic disorders post-partum raised the risk of extended lactations.
• High ambient temperature around the time of first services raised the risk of extended lactations.
• Odds of extended lactations are about 4 times lower in primiparous cows than older cows.

Two data files consisting of 3278 (one herd) and 4393 lactations (three herds) were used to characterize milk production of high-yielding Holstein cows milked three times per day and experiencing extended lactations (up to 1399 days) in a hot environment. Additional objectives were to identify risk factors for the occurrence of extended lactations, to assess the association between lactation length and milk yield and to determine the conception rates of cows with extended lactations and with multiple services. Thirty percent of lactations were between 450 and 1399 days. Lactation yield of cows with lactations >900 days was over 30,000 kg. Average daily milk yield from 305 to >450 days in milk was 29.9 and 31.9 kg for primiparous and multiparous cows, respectively. The epidemiological analysis of risk factors for extended lactations using a multiple variable logistic regression indicated that ketosis (OR=1.4), peak milk yield (<50 vs >50 kg d−1, OR=1.4), temperature-humidity index at 60 days postpartum (<82 vs >82 units, OR=1.4), retained placenta (odds ratio (OR)=1.5), 305-d milk yield (<11,000 vs >11,000 kg, OR=1.6) and the occurrence of metritis (OR=1.8), significantly increased the risk for lactations >450 days. Primiparous cows had less than half the risk of extended lactations (OR=0.3) compared to multiparous cows. Conception rate in cows with extended lactations decreased as number of services increased (conception rate=50.5% for 4 services and 13% for ≥14 services). The data showed that well-managed Holstein cows milked three times daily were capable of lactating for over 1200 days with remarkably high persistency and with high milk yield at drying-off. Reproductive and metabolic disorders associated with calving were important risk factors for extended lactations.