Nikola Raguž

Methods for estimation of daily and lactation milk yields from alternative milk recording scheme in Holstein and Simmental cattle breeds.

Abstract 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.

Genetic evaluation for longevity of Croatian Simmental bulls using a piecewise Weibull model

Abstract. Genetic evaluation of sires based on length of functional productive life of their daughters was carried out using a Weibull proportional hazard sire-maternal grandsire model. The data consisted of 214634 registered Slovak Simmental cows with censoring of 21.34%. Besides the random sire and maternal grandsire effects, the model included time dependent effects parity × stage of lactation interaction, within-herd standard deviations of milk production, herd × year × season interaction, change of herd size with respect to the previous year, and time independent effect age at first calving. Within-herd, the standard deviations of milk production had the most important influence on functional productive life. The highest risk of culling was found in cows with the lowest milk production class (uncompleted lactation). Risk of culling decreased with an increasing milk production class. Estimated heritability of functional productive life was 0.05 on the original scale. Breeding values of sires expressed as a risk ratio of their daughters were between −0.57 and 0.53.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 lowestmilk yields were at approximately 2.7 times higherrisk 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.

Influence of temperature-humidity index (THI) on daily production of dairy cows in Mediterranean region in Croatia

With the aim of analysis the influence of temperature-humidity index (THI) on daily performance of dairy cows in Croatian Mediterranean region, 217,509 test-day records collected from January 2005 until April 2010 were extracted from HPA (Croatian Agricultural Agency) database. Milk recording in Croatia occurs according to the alternative milk recording method every four weeks, therefore, daily production of milk yields and components was projected using projection parameters estimated in authors’ earlier research (Gantner, 2008). Logical control of data was performed according to ICAR standards (2003). The yields recorded after the 500th lactation day was deleted from the dataset. According to the parity, cows were divided into three classes (heifers, cows in second lactation and cows in third and higher lactations) while, regarding the calving date, cows were divided in two calving season subgroups. According to the test date, four measuring season subgroups were created. Based on temperature and relative humidity recorded in stable at each milking control the daily temperature-humidity index (THI) values were calculated using the equation by Kibler (1964). For estimation of the influence of exceeded THI (above critical value -72) on daily production of dairy cows statistical model that take into account effects of lactation stage, breed, calving season, measuring season as well as effect of THI group (T1 - THI under critical value; T2 - THI above critical value). The significance of differences between the means of the daily milk, fat, protein and lactose yield, as well as between the means of the daily fat, protein, and lactose content within the THI classes in regard to parity classes was tested with Scheffe test. For the statistical analysis and the figures drawing the SAS/STAT package was used (SAS Institute Inc., 2000). The exceeded THI significantly decrease daily milk, fat, protein and lactose yield in heifers and cows in third and higher lactations. In second lactation determined decrease was not significant. Highly significant decrease of daily fat and protein content was determined in all cows, regardless the parity. The daily lactose content was not significantly influenced by enhanced value of THI. Management strategies are needed to minimize heat stress and to attain optimal cows’ production in climatic condition of Croatian Mediterranean region.