Effect of Insemination Time in Pre- and Post-Milking on Pregnancy Rate of Isfahan Holstein Cows

 The main aim of dairy farms is to increase the economic milk production of each cow in each lactation period. Therefore, the effective genetic and environmental factors should be identified and evaluated. Today, the main cause of economic losses to dairy herds is low reproductive efficiency. Any factor that causes delay or non-pregnancy of dairy cows leads to reduced milk production and calf birth. Stress is known as a major factor in reducing of fertility of dairy farms. Therefore, it is necessary to find management solutions that can reduce the side effects of this factor. The objective of this study was to evaluate the effect of insemination in pre- and post-milking time on pregnancy rate of Holstein Cows in Isfahan province. 

This research was conducted in the winter season between 2015 and 2020 in two large industrial herds in Isfahan. 2340 Holstein cows with 1 to 5 lactations were studied. The cows were randomly divided into two equal groups. According to the time of estrus observation and calculation of insemination time, half of the cows (1170 heads) were inseminated during 2 hours after milking and the other half during 2 hours before milking. Statistical analyzes were done using SAS software version 9.4. Descriptive statistics were estimated for enumerated data with frequency procedure (Proc FREQ) and for continuous data with mean procedure (Proc MEANS). Logistic regression method (Proc GLIMMIX) was used to analyze the results of insemination (1 = lead to pregnancy and 0 = no lead to pregnancy). The final model was including the fix effects of herd-year-season, insemination time compared to milking (pre- and post-milking), age at insemination time (2 levels including less equal and more than 40 months), MIM (nine levels), interaction effect of insemination time and age, interaction effect of insemination time and milk yield level (low and high producing) and the random effect of semen as bulls.

Odds of pregnancy of inseminated cows in the post-milking time was more than inseminated cows in the pre-milking time (1.5 times). Also, in the group of high producing cows (with a milk average yield of more than 51 kg) and low aging cows group (less equal 40 months), the odds ratio of pregnancy of post-milking insemination was estimated higher than pre-milking insemination (1.6 and 1.85, respectively). Comparison of least squares mean (LS Means) of pregnancy rate in cows based on the time of insemination showed that in generally there is a 0.09 pregnancy difference between insemination time in pre- and post-milking. The mean of pregnancy rate in pre- and post-milking insemination was 0.31 and 0.40, respectively. Due to be significant of the odds ratio in the group of high producing cows (with a milk average yield of more than 51 kg), comparison of least squares mean of pregnancy rate in insemination times showed a difference of 0.1 in this group. The mean of pregnancy rate of high producing cows in pre- and post-milking insemination was 0.30 and 0.40, respectively. Also, due to be significant of the odds ratio in the group of low aging cows (less than equal to 40 months), the difference of least squares mean of pregnancy rate at insemination times was estimated 0.15. In other words, the mean of pregnancy rate of low aging cows in pre- and post-milking insemination was 0.34 and 0.49, respectively. For low producing (less than 51 kg) and high aging cows (more than 40 months), although the mean of pregnancy rate at insemination time of post-milking was numerically higher than pre-milking, no significant difference was observed. Although there is a little information for the physiological factors of higher pregnancy in cows that have been inseminated in post-milking, it seems that reducing stress and consequently lower cortisol concentrations due to discharging milk of udder can be an effective factor. 

The results of this study showed that the time of insemination based on milking time makes a significant difference in the pregnancy rate of Holstein cows. It was also found that high producing cows with a milk average yield of more than 51 kg and low aging cows with age of less than equal to 40 months had the maximum chance of pregnancy at the time of insemination of post-milking due to reduced effect of milking stress at insemination time. Although the physiological reasons need to be carefully investigated, the findings of this study can be recommended as a suitable solution for insemination of low aging and high producing cows to increase herd fertility.