Lactation Persistency in Spring‐Calving Grazing Dairy Cows During Extended Lactation

Lactation persistency, or the ability of a cow to maintain high milk yield throughout lactation, is important for extended lactation (EL: beyond 305 days in milk). Persistency is a characteristic that could support the identification of cows suitable for EL. This study evaluated the factors that affect lactation persistency during EL in grazing, spring calving dairy cows. A total of 4599 EL (460‐to‐820‐day calving intervals) and their associated herd tests were modelled using random regression with a fifth‐order Legendre polynomial. Persistency was calculated as the total yield of the last third of the lactation divided by the total yield of the first third of lactation, expressed as a percentage. Mean (standard deviation) persistency of milk and milksolids (fat + protein) yields for a 550‐day lactation were 61% (15.3) and 73% (17.4), respectively. Parity had a significant effect on both milk and milksolids persistency, decreasing from parity one to parity four or greater, while breed had no effect. Variability of persistency, irrespective of cow factors, supports it as a promising selection characteristic, alongside milk production, for cows suitable for an EL system under grazing conditions. Both high production and lactation persistency are likely important determinants of a successful and profitable system.