Diphasic analysis of lactation curves in dairy goats

Milk production data on 5084 Alpine, 2052 LaMancha, 7024 Nubian, 2194 Saanen, and 2339 Toggenburg does were grouped into 90 subclasses: five breeds x three parities (1, 2, and 3) x two seasons of kidding (early, December to March; late, April to July) x three measures of 305-d milk production within breed (low, medium, and high). Subclass means of milk production for 100 3-d groups were smoothed and used to estimate parameters of a diphasic function, which is the sum of two logistic functions. Characteristics for each phase of the lactation curve (initial, peak, and 305-d yields, time of peak, and duration of phase), which are functions of parameters of the diphasic function, were then analyzed using a linear model including breed, parity, season, and mean of measure of production as â covariate, weighted by the number of observations in each subclass. Breed had little effect on the shape of the lactation curve in dairy goats. Parity affected primarily characteristics of the second phase of lactation. Season of kidding had the most consistent effect on the lactation curve: affecting characteristics of each phase. Measure of production affected characteristics of the second phase more than those of the first phase. First phase, with its proximity to overall peak and short duration, could be interpreted as a "peak" phase. Second phase, affected largely by parity, could be interpreted as a "persistency" phase.