Modeling Lactation Curve in Primiparous Beef Cattle

The work describes lactation curves and compares two methods to estimate milk yield (MY) in a grazing beef cattle herd of the EEBR Station-Udelar, Uruguay. Twenty-four Hereford, Angus and F1-crossbreed primiparous cows were used to estimate MY once a month, from birth to weaning, by weigh-suckle-weigh (WSW) technique and milking-machine (MM). Milk yield (MY), milk yield retained energy (ReMY), and calf weight were analyzed as repeated measures in a model including- sex of calves, month of lactation, cow and calf breed, milking method, estimation day (1 or 2), and post-partum days as fixed effects, and cow nested within breed as the random effect. The correlation analysis and the Gage r&R coefficient (repeatability and reproducibility) between the two methods were used to study their associations. Lactation curves were compared (AICC and BIC) using Wood (1964), and Jenkins and Ferrel (1984) models. The MY estimated differed with the methodology being WSW higher than MM (P < 0.001). The r&R coefficient (0.83) suggest lower associations between WSW and MM, being 18% and 6% the coefficients of variation, respectively. Cow breed was not significant for MY. Calf live weight and ReMY were negatively associated (-0.52, P < 0.0001). Based on variability observed, MM is more accurate to estimate MY and Wood curve the most adjusted to describe lactation in grazing beef cattle.