Fitting lactation data with two mathematical models and extension factors for milk, fat and protein of Belgian dairy goats

peer reviewedData for 18 155 test day milk yields and fat and protein percentages recorded from 15 February 1988 to 1 September 1993 were obtained from the Office de promotion des petits élevages en Wallonie. Due to irregular test intervals and a variable number of tests per lactation, production was estimated at 25 day intervals (25, 50, ..., 250 days). A total of 13 773 test day records for Anglo-Nubian, Chamoisee, Saanen and crossbreds were available for analysis. Parities were classified into 1 and ≥ 2. The inverse polynomial and the incomplete gamma functions were fitted to test day milk yields in order to define the shape of the lactation curves for the various breeds and parities. Two data subsets were created by random selection of entire lactation data from the original data set, and both functions were fitted to test day milk yields within breed-parity classes. Parameters of equations estimated on a subset of the data were validated on the other subset. Estimates of peak yields were higher and times of peak yield later by the inverse polynomial method than by the incomplete gamma but remained within ranges found in the literature. Based on the coefficient of multiple determination (R2), both equations were equally accurate in fitting lactation data of a subset. Though average residual deviations were slightly higher with the inverse polynomial than with the incomplete gamma, the crossvalidation did not reveal any particular trend of residuals for any equation. For practical reasons, extension factors for milk, fat and protein yields were derived using the inverse polynomial and are presented

Effects of parity, breed, herd-year, age, and month of kidding on the milk yield and composition of dairy goats in Belgium

Lactation records, collected from 1989 to 1995, for Belgian dairy goats were examined for effects of parity, breed, herd-year, age at kidding, and month of kidding on milk, fat and protein yields, and on fat and protein percentages. Total lactation yields were computed from test-day yields, using Fleischmann's method. For lactations longer than 250 days, yields were truncated at 250 days. Partial lactations (25 days minimum) were extended to 250 days, and fat and protein percentages were deducted from the yields. After editing a total of 2997 lactation records of 2562 does remained. The parity classifications were first and second or greater; kidding seasons were defined as December, January, February, March, April to July, and August to November. The breed groups were Anglo-Nubian, Chamoisee, Saanen, and crossbreeds. The data were analysed within parity with a fixed-effect model that included effects of herd-year, breed, month of kidding, and age at kidding. All effects in the model were significant on the traits measured for parities, except the age effects on fat percentage and of month of kidding on protein percentage at first parity. Saanen had the highest milk yield at first parity; at later parities, the Chamoisee produced most. Yield increases were large between first and later parities. Does freshening in February produced more milk than those that freshened at other months. Monthly variations in fat and protein percentages across parities, although mostly significant, were small. Yields increased with age at first parity; for later parities, they increased until 25-30 months for milk and until 40 months for fat and protein, and then decreased. Fat and protein percentages increased up to 15-17 months of age, and then decreased to the lowest values around 25 months; this was followed by an increase around 32 months, which tended to stabilize until 50 months. The ranges of R2 associated with effects were: for herd-year, 43-52% and 22-36%; for breed, 3-11% and 4-12%; for month of kidding 6-10% and 2-5%; for age at kidding 3-7% and 1-17%-for first and second, and later parities, respectively. © 1997 Blackwell Wissenschafts-Verlag, Berlin