The effect of non-genetic factors on milk yield and composition in goats of White Short-Haired breed.

摘要

The objective of the paper was to determine the effect of some non-genetic factors on milk yield and composition in goats of White Short-Haired breed (WSH); their population is largest in Slovakia. Another objective was to determine differences in the production and content of basic milk constituents in the goat population concerned using a suitable model of the analysis of covariance, in relation to lactation number and number of sucking kids. A total of 4950 lactations of goats from 64 farms wereassessed; data were acquired as part of milk records (MR) in 1995-1997. All records of WSH goats were excluded from the analysis if milking period (MP) was shorter than 150 days and longer than 325 days, if the interval between parturition and 1st milkcontrol (INT) was shorter than 10 days and longer than 70 days, and if the actual milk yield of goats was lower than 100 litres and higher than 1300 litres. Milk yield was measured after kid weaning. Other data excluded from the analysis concerned fat content lower than 2% and higher than 5.5%, average protein content lower than 2% and higher than 4% and average lactose content per milking period lower than 2% and higher than 5%. The least-squares method and several linear models with fixed effects wereused for estimation of the effect of environmental factors in adjusted data from MR. The effects of these factors: farm (F), lactation number (LN), check year (Year), season, i.e. month of parturition (Se), number of sucking kids (NSK), and of compositevariable FYS (F x Year x Se) and 2 accompanying variables (MP, INT) were estimated, as exerted on actual and standardized milk yield (AMY, SMY) and on fat, protein and lactose contents (%F, %P, %L). Standardized yield was calculated per 240-day milkingperiod. The highest determination coefficients (R2) were calculated in models with composite variable FYS; they were substantially higher for AMY and SMY (0.76-0.79) than for %F, %P and %L (0.38-0.47). If factor F was excluded from the analysis of covariance, the fall of R2 values was steep (in comparison with the other analysed factors) for both the production and milk constituents. Factors F, LN, Year, Se, and FYS had a significant effect (P < 0.001) on AMY and SMY, and a considerable effect on %F, %Pand %L. The significance of the effect of factor NSK on milk yield and composition was related to other analysed environmental factors to a large extent. The effect of the latter factor on AMY was statistically significant (P < 0.05). Neither linear norquadratic effect of MP and INT on milk yield was observed. Variability in MP was conditioned by factors Year, F and Se, being 214 days on average with a range from 151 to 314 days. In the population of WSH goats, actual milk yield after adjustment of systematic environmental effects (LSM) amounted to 396.9 litres, SMY 450.7 litres, fat content 3.54%, protein content 2.83% and lactose content 4.47%. Goats in the 1st lactation (AMY 378.3 litres, SMY 428.4 litres) produced less milk than goats in higher lactations (P < 0.001). Actual and standardized milk yield of goats in the 2nd lactation was also lower (P < 0.001) than that of goats in the 3rd and 4th lactations, when milk production was at a maximum level. The highest SMY in milking period was achieved also by goats that nursed triplets (493.3 litres). In comparison with goats nursing single kids or twins till weaning this difference was significant (P < 0.05). Goats in the 1st lactation had a higher (P < 0.001) content of proteins (2.92%) than goats in the 3rd lactation (2.86%) and in the other lactations (P < 0.01). The number of sucking kids during nursing did not have a significant effect on goat milk composition. All the data can be used to draw up proposals of new methods of breeding value estimates in WSH goats that should be used for selection aimed at faster genetic progress in milk and utilizable dry matter production.