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Modeling Extended Lactation Curves in Dairy Goats Using Grafted Polynomials

T. A. Gipson1 and G. R. Wiggans2

1E (Kika) de la Garza Institute for Goat Research, Langston University, Langston, OK, 2Agricultural Research Service, USDA, Beltsville, MD

Some dairy goat producers milk their does for an extended period before rebreeding and drying off, but little information is available about the shape of extended lactation curves for goats. To examine the effect of extending lactations upon the shape of the lactation curve, test-day records (n = 7,425) of 469 does with extended lactations were obtained from USDA/AIPL. Range for days in milk was 419 to 696 d, with an average of 530 d. The number of test days ranged from 12 to 20 with an average of 16 per lactation. Lactations from six breed types (Alpine, LaMancha, Nubian, Saanen, Toggenburg, and Experimental) and four parities (first, second, third, and fourth or greater) were included. Nonlinear regression analysis using several grafted polynomial models was conducted for each animal. The models used were quadratic-linear (QL), quadratic-quadratic (QQ), quadratic-quadratic-linear (QQL), quadratic-quadratic-quadratic (QQQ), quadratic-quadratic-quadratic-linear (QQQL), quadratic-linear-quadratic-quadratic (QLQQ), and quadratic-linear-quadratic-quadratic-linear (QLQQL). The most appropriate model was selected by testing for significant (P<.05) reduction in error sums of squares from the QL model for each individual animal. A (P < 0.01) higher percentage of selected models were QL than were QQQ (64 vs 22%, respectively). The selection of other models was 6% for QQQL, and 3, 3, and 2% for, QLQQ, QQ, and QQL, respectively. The QLQQL model was not selected. There were no significant (P > 0.10) breed or parity effects on the distribution of selected models. The median join point was 300 for QL, and 348 for QQ. Points were 193 and 343 for QQQ; 87 and 147 for QQL; 445, 331, and 131 for QQQL; and 287, 185, and 57 for QLQQ. A QL grafted polynomial model appears sufficient to model the majority of extended lactation curves in dairy goats.


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