Search

In order to determine the effect of breeding region on differences in persistency of heat stress effect in first parity Simmentals test-day records provided by the Croatian Agricultural Agency were analysed. Only cows with detected statistically significant decrease in daily milk yield at set temperature-humidity index (THI) threshold value (65, 70 and 75) were included in the further analyses. The persistency of heat stress effect regarding the daily milk traits was determined as a drop in the subsequent milk recordings (1 st and 2 nd). The research results indicate significant difference in animals' response to heat stress effect due to breeding region and individual's susceptibility to heat stress. The most pronounced and persistent negative effect of heat stress was determined in cows reared in Eastern region. Also, the negative effect of heat stress was more pronounced and more persistent in Simmentals that were heat stressed at the lower THI threshold values (cows that are more susceptible to heat stress).

In order to estimate the variance components as well as the breeding value for daily milk production of Holstein cows in terms of heat stress defined as different THI threshold values (68, 70, and 72) 1,275,713 test-day records from 90,159 cows were used. Temperature-humidity index (THI) calculated from ambient temperature and relative humidity recorded in the barns on the milk recording day was used as the measure of heat stress. Daily milk production was analysed as a bivariate variable, in the normal conditions (under the set THIthreshold value), and in terms of heat stress (above the set THIthreshold value) using a model that accounted variances due to lactation stage, calving season, age at calving, breeding region, parity, permanent environment, interaction between hear and year, and individual animal. The estimated high genetic correlations between the daily milk production in normal and heat stress conditions, as well as high correlations among the EBVs in normal and heat stress conditions, point out that Holstein cows with high productivity in normal condition will sustain an alike level of production even in terms of heat stress. These results indicate that the Holstein cows are very well acclimatized to the environmental conditions at dairy farms in Croatia. However, these findings should be taken with caution, given that measurements of microclimate parameters were conducted only once at a milk recording day, so the reliability of such measurements could be questionable. In conclusion, in order to realize a completely unambiguous conclusion, further studies that include multiple daily measurements of microclimatic parameters in the farms facilities are necessary. [ABSTRACT FROM AUTHOR]

Variance components for daily milk production as well as breeding value of dairy Simmentals for daily milk production in terms of heat stress defined as different values of THI threshold values (72, 74, and 76) were estimated using 1,636,192 test-day records provided by the Croatian Agricultural Agency. Temperature-humidity index (THI) calculated from ambient temperature and relative humidity recorded in the barns on the milk recording day was used as the measure of heat stress. Bivariate estimation model accounted variances due to lactation stage, calving season, age at calving, breeding region, parity, permanent environment, interaction between hear and year, and individual animal. Obtained high genetic correlations between the daily milk production in normal and condition characterized by heat stress, as well as very high correlations between the EBVs in normal and heat stress conditions indicate animals' high level of acclimatization to the environment on dairy cattle farms. Since microclimate measurements were carried out only once at a milk recording day, these results should be taken with caution. Generally, further research with multiple daily measurements of the microclimate parameters in the facilities is necessary to provide a fully unambiguous conclusion. [ABSTRACT FROM AUTHOR]

Copyright of Züchtungskunde is the property of Verlag Eugen Ulmer and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In order to determine the persistence of heat stress effect in dairy cows regarding the breed, parity and susceptibility to heat stress, over 1.9 million of test-day records from Holstein and Simmental cows reared in Croatia were used. The persistence of heat stress effect in the subsequent milk recordings was determined in cows that had significant decrease of daily milk yield at different THI threshold value (65, 70, 75, 80, and 85). The obtained results indicate that cows heat stressed at lower THI threshold value had higher proportional drop of daily milk yield in subsequent milk recordings. Also, primiparous and Simmentals experienced higher proportional drop when compared to multiparous and Holsteins. The negative effect of heat stress on daily milk contents that existed in all cows was more pronounced in the 2nd subsequent milk recording. The determined increase of somatic cell count in more heat stress sensitive Holsteins (THI in 65, 70, 75) indicates that Holstein cows, in terms of heat stress, also become more susceptible to mastitis. Finally, the lower was the THI threshold value, the stronger and more persistent was the heat stress effect. Considering the quite probable increase of the frequency of heat stress days, it is necessary to develop an adequate strategy for dairy farming. Aiming the accurate answer to the problem of heat stress in this region, it is necessary to conduct further research on genetic evaluation. [ABSTRACT FROM AUTHOR]

Considering the rapid climate changes worldwide, with the purpose to reduce financial losses for dairy farmers and enable a more sustainable farming as well, there is an increasing necessity to implement breeding values for heat resistance in breeding strategies. The estimation of breeding values requires a determination of temperature-humidity index (THI) threshold value. Therefore, the objective of this study was to determine the temperature-humidity index (THI) threshold values for daily milk yield, daily fat and protein content of milk produced by Holsteins and dairy Simmentals in Croatian farms. With that purpose, 1.070,554 test-day records from 70.135 Holsteins reared on 5.679 farms and 1.300,683 test-day records from 86.013 Simmentals reared on 8.827 farms in Croatia were compared to records of ambient temperature and relative humidity collected in the barn, during regular milking recordings from January 2005 to December 2012. Accordingly the following conclusions could be drawn: threshold values for daily milk yield were highly dependant on the parity and breed (Holsteins: 68, 69 and 72 for 1st, 2nd, 3+ parity; Simmentals: 77 for 1st); daily fat and protein contents showed a highly significant decrease due to heat stress condition (THI in 66-80) in both, Holstein and Simmental cows, and in all parity classes; multiparous cows were more resistant to heat stress than primiparous; Simmental cows were more resistant to heat stress than Holsteins. [ABSTRACT FROM AUTHOR]