Genetic evaluation of sires for functional longevity was conducted using survival analysis techniques. The data set consisted of 49 659 Simmental cows with first calving from 1997 to 2008. A piecewise Weibull sire model was used to estimate breeding values of 251 bulls for functional length of productive life of their daughters. The model was stratified by parity i.e. a separate baseline hazard was computed for each stratum. Besides the random sire effect, the model included the fixed time independent effects of age at first calving, herd size and region as well as the time dependent effects of relative milk production and year*season of first calving. The highest impact on longevity was found for relative milk production. Cows with the lowest milk yields were at approximately 2.7 times higher risk of culling compared to cows with average milk production. Effects of age at first calving, herd size and country region had lower impact on longevity. Sire variance was 0.023 which results in a heritability of 0.06 for functional length of productive life. The average approximate reliability of estimated breeding values was 0.49. Genetic trend showed no clear tendency by year of birth of bulls.
This study was focused on genetic evaluation of longevity in Croatian Simmental cattle using linear and survival models. The main objective was to create a genetic model that is most appropriate to describe the longevity data. Survival analysis, using piecewise Weibull proportional hazards model, used all information on the length of productive life including censored as well as uncensored observations. Linear models considered culled animals only. The relative milk production within herd had a highest impact on cows' longevity. In comparison of estimated genetic parameters among methods, survival analysis yielded higher heritability value (0.075) than linear sire (0.037) and linear animal model (0.056). When linear models were used, genetic trend of Simmental bulls for longevity was slightly increasing over the years, unlike a decreasing trend in case of survival analysis methodology. Average reliability of bulls' breeding values was higher in case of survival analysis. The rank correlations between survival analysis and linear models bulls' breeding values for longevity were ranged between 0.44 and 0.46 implying huge differences in ranking of sires.
The aim of this study was to compare different statistical methods for the estimation of daily and 305-day lactation milk, fat and protein yields of Holstein and Simmental cattle breeds using an alternative milk recording scheme. Data included 6,824 individual test-day milk yield records collected according to the A4 milk recording method on 668 cows reared on 15 family farms. Daily milk, fat and protein yields were estimated using several statistical methods with regard to breed. The 305-day lactation yields were calculated from estimated daily yields using the Test Interval Method. The correlation between estimated and true yields, as well as the mean difference among estimated and true yield were used as the evaluation criteria for estimation methods. The linear regression of daily to partial milk, fat and protein yields while taking into account the interval between successive milkings was shown to be the most accurate model for estimating daily values, either from morning or evening records. The simple doubling of morning or evening records overestimated and underestimated the daily yields, respectively. When 305-day lactation milk, fat and protein yields were compared no difference between evaluated methods were found. Also, a separate estimation of daily and 305-day lactation yields according to breeds did not result in increased estimation accuracy
The objective of this study was to determine the effect of various milk recording methods (A4, AT4, A6, AT6) on prediction accuracy of 100, 200 and 305-day milk yield. The data used in this study were 11,430 individual test-day milk yield records collected from November 2004 to November 2006 on 813 cows reared on 15 family farms in Croatia. Milk recording was performed according to A4 and A6 milk recording method by the field officer of the Croatian Livestock Centre. From the corrected database with test-day records, two different datasets were created. The first dataset (A4; n = 7,500) included test day records collected every four weeks, while the second dataset (A6; n = 3,830) included test day records collected every six weeks. When lactation milk yields were predicted from alternative milk recording methods (AT4, AT6), daily (24 h) milk yield was estimated from single evening or morning milk yield using linear model that taken into account effect of interval between successive milkings. Lactation milk yield was calculated for three different days in milk (100, 200 and 305 days) using the Test Interval Method (TIM) that is reference method by ICAR (ICAR, 2003). Results show that the alternate milk recording method at 4-week intervals provides low bias and high accuracy of prediction of 100, 200 and 305-milk yields, while milk recording methods at 6-week intervals gives prediction of 305-milk yield with higher bias and lower accuracy.
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