چكيده انگليسي :
In this study, to estimate genetic parameters of the trait time to peak lactation and its relationship with the trait persistence in Holstein dairy cows, 8,003,232 records belonging to 147 Holstein dairy herds in Isfahan province, Iran, collected between 2011 and 2022, were used. Information such as the average somatic cell score, minimum somatic cell score, 305-day fat yield, 305-day protein yield, peak milk yield, peak fat percentage, peak protein percentage, time to peak lactation, days in milk, longevity, and productive life length was extracted. The CFC software was used to examine the pedigree structure, and SQL version 18 was used to edit the data. The lactation curve parameters of the cows, as well as the time to peak lactation, 305-day milk yield, and days in milk, were estimated using the Wilmink function. The results of the descriptive statistics of the traits showed that the time to peak lactation and days in milk had means of 63.59 and 94.71, and variances of 223.28 and 13.09, respectively. The peak milk yield had a mean of 46.55, a variance of 73.61, and a coefficient of variation of 18.43%. The results of this study showed that the correlation between the time to peak lactation and peak milk yield in the first and second lactation was -0.92 and -0.80, respectively, indicating a strong negative correlation. However, a moderate negative correlation was observed between the time to peak lactation and peak milk yield in the third lactation (-0.54). Additionally, a positive and strong correlation was observed between the time to peak lactation and days in milk in the first four lactations. The time to peak lactation was negatively and weakly correlated with longevity and productive life length in all lactations. The results of this study showed that the heritability of 305-day milk yield was 0.11, and the repeatability was 0.16. The highest heritability was observed for peak fat yield (0.49), and the lowest heritability was observed for average somatic cell score and 305-day fat yield. Both average somatic cell score and 305-day fat yield had the same heritability of 0.04, but the repeatability of 305-day fat yield was higher at 0.10. The genetic trend of traits showed a negative trend from 2011 to 2012 and a positive trend from 2012 to 2013. From 2013 to 2016, a negative trend was observed, and from 2016 to 2018, a positive trend was observed. Also, from 2018 to 2019, no significant changes were observed in the trend. However, from 2019 to 2020, a negative trend was observed. The trait of time to peak lactation had a negative correlation with milk yield, protein yield, and fat yield, with the highest negative correlation (-0.81) with milk yield. It also had a positive correlation with minimum somatic cell score and days in milk. Additionally, the genetic correlation of milk yield with protein yield, minimum somatic cell score, days in milk, longevity, and productive life length was negative. In this study, no environmental correlation was found for 305-day protein yield with protein yield, 305-day milk yield with days in milk, and productive life length with 305-day milk yield. Overall, it can be concluded that with an increase in the number of lactations, the positive effect of time to peak lactation on overall days in milk decreases, but it still remains positive in all lactations. Therefore, a delay in reaching peak lactation leads to a longer lactation length, but this effect gradually weakens with an increase in the number of lactations. In all lactations, the effect of time to peak lactation on longevity and productive life length was almost zero and negligible. In other words, the time to peak lactation does not have a significant effect on the productive life of animals, and this effect does not change with an increase in the number of lactations.
