ELSEVIER Small Ruminant Research 18 ( 1995) 75-79 zyxwvutsrqponmlkjihgfedcbaZYXWVU Small Ruminant Research A non-linear modification of Wood’s equation fitted to lactation curves of Sardinian dairy ewes A. Cappio-Borlino, G. Pulina”, G. Rossi lsriruto di Zootecnica. Facoltti di Agraria, Universitci di Sassari, Via de Nicola, 07100 Sassari, Sardinia, Italy Accepted 20 January 1995 Abstract Wood’s equation, which is a widely used mathematical model for lactation curves, is not sufficiently flexible in situations frequently found in dairy sheep. For this reason a modified non-linear (y=u~p(-“‘) ) version was tested on the individual production (expressed in grams) of 64 Sardinian dairy ewes with differing production levels, type of lambing and udder health. The results showed that the modified version of the model (a = 702.3 + 56.2,b = 1.29 It 0.09, c = 0.133 + 0.013) fits the lactation curves well (R* = 0.905; residual standard deviation 145.3) with few interactions required for convergence ( < 5). Production level and type of lambing influenced all parameters, whereas udder health influenced only the a parameter. Keywords: Lactation curves; Dairy ewes; Non-linear equation 1. Introduction Despite the existence of models which fit lactation curves well and which have a high biological basis (Cobby and Ledu, 1978; Neal and Thornley, 1983; Morant and Gnanasakthy, 1989; Gipson and Grossman, 1990; Stanton et al. 1992; Ferguson et al., 1993), Wood’s model ( 1967) is still widely used (Rowland et al., 1982; Fade1 et al., 1989) including ovines (Tor- res-Hernandez and Hohenboken, 1980; Aguilar et al., 1989; Cappio-Borlino et al., 1989; Campos et al., 1991; Sakul and Boylan 1992; Caja et al., 1992) thanks to its simplicity as a first approach useful to roughly deter- mine some differences such as high and low milk pro- duction (Pulina et al., 1993). However, Wood’s model has been criticised because it underestimates peak yield and overestimates yield in the decay phase (Cobby and Ledu, 1978; Lewis Johnson and Lang Tran, 1990). In * Corresponding author. Tel. 079/229305. Fax 079/229302. the specific case of the Sardinian ewe, which because of its wide existence (3.5 million head) is considered the Italian milk breed, determination of the lactation peak and other characteristics of the curve (rapidity in achieving the peak and persistence) is very important for genetic and feeding purposes. This is why the adop- tion of a model which, even if not too precise has parameters directly related to these aspects, is impor- tant. Also, in dairy sheep breeds more complicated situations frequently arise and in these cases this model loses its biological meaning. An example is when milk production dramatically falls immediately after the peak of lactation due to inadequate nutrition or general adverse environmental conditions (Casoli et al., 1989; Gargouri et al., 1992), or when information is lacking regarding the early stage of lactation due to the suckling of lambs during the first 4 or 5 weeks (Gabina et al., 1993; Giaccone et al., 1993). In these cases the zyxwvutsrq b parameter in the non-linear form of Wood’s equation, 0921-4488/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO921-4488(95)00713-X