On the use of simple ratios between lactation curve coefficients to describe parity effects on milk production

The objective of this study was to quantify how the pattern of milk production relative to time from calving is affected by parity in cows fed high quality rations. For this purpose two models; those of Emmans and Fisher (1986) and Dijkstra et al. (1997), were considered. Comparison with Wood's (1967) function was made to evaluate their fitting ability. Daily records of milk yield from 40 cows fed a grass silage based, high concentrate total mixed ration were used. The cows had ad libitum access to food, they were milked twice daily. Each of the cows had milk yield records from calving to 240 days post calving in parities 1, 2 and 3. The model of Dijkstra et al. (1997) was found, on inspection, to be an alternative parameterization of the Emmans and Fisher model (1986). In the analyses, the Emmans and Fisher form was used: Yield = all exp [-c(days from calving)] where U = exp {- exp (G(0) - b[days from calving])}. The Emmans and Fisher model (1986) performed marginally better than the Wood's function (1967) in terms of percentage of variance accounted for and residual standard error. There was no significant effect of parity on G(0) or b, which are the main parameters describing the evolution of lactation to peak. However, there was a highly significant effect (P < 0.001) of parity on coefficients a, the scalar, and c, the decay coefficient. The coefficient values in parity 1 and 2 were found to be a constant proportion of the values in parity 3. Parity effects can therefore be described by simple ratios, offering the possibility of simplifying the inputs needed in models to describe potential milk production. (C) 1999 Elsevier Science B.V. All rights reserved.