Submitted to: American Dairy Science Association Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 7/24/2001
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: 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 this, test-day records (n = 7425) of 469 does with extended lactations were obtained from USDA/AIPL. Range for days in milk was 419d to 696d with an average of 530d. 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 significantly (P<.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>.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; 131, 331, 445 for QQQL; and 57, 185, 285 for QLQQ. A QL grafted polynomial model appears sufficient to model the majority of extended lactation curves in dairy goats.