LACTATION CURVES IN ANGUS, BROWN SWISS AND THEIR CROSSBRED COWS: II. COMPARISON OF EQUATIONS

Abstract

In beef cattle production systems it is desirable that the milk produced by the cow is acceptable, that is, enough to allow a good growth of her calf without affecting her reproduction. For that purpose, it is important to use the equation that best predicts the lactation curves. The objective of this study was to compare eight equations to estimate parameters of lactation curves in cows of Angus (A), Brown Swiss (P) and their reciprocal crosses AP (A x P, and P x A). Milk production data (297 lactations) were taken weekly for 180 days postpartum, using the weigh-suckle-weigh technique. The following equations were evaluated for each lactation: 1) Yt = atbe-ct; 2) Yt = ae-ct; 3) Yt = t/ae ct; 4) Yt = ae(bt+ct²); 5) Yt = a + bt; 6) Y t = a + bt + ct 2; 7) Yt = a - bt - ae-ct, and 8) Y t = t/(a + bt + ct 2). Equations 5 and 6 were estimated using least squares linear regression; the others using least squares nonlinear regression. The residual mean squares (CMR) were used as a criterion to evaluate the estimation of the curves. All eight equations were compared and ranked using Friedman?s ranking test. Equation 1 was very flexible because it fitted well the lactation curves of different genotypes and lactation numbers. However, the equation with the best fit depended on the genotype of the cow. Equations 1 and 8 fitted best lactation curves (p