الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
This study included two experiments. The first experiment was conducted on 920 purebred multiparous Holstein cows and lasted for four years (2015-2018). The first one aimed at studding the effect of the heat stress through the temperature and humidity index (THI), which were divided into three categories: high, medium and low on the reproductive traits of the Holstein cows. The results of this experiment indicated the existence of a significant negative effect of the high THI category on the reproductive traits, namely: service per conception, the embryo loss, pregnancy rate, and abortion rate, compared to those in the medium and low categories. After the end of the first experiment, the second one was started on 60 pure Holstein cows in order to study the effect of two folic acid (FA) doses by drenching on reducing the negative effect of heat stress (high THI) on reproductive performance during the first month of pregnancy for Holstein cows. The tested cows were divided randomly into three tested groups: the first group (control, without drenching) and the second group were dosed at a rate of 5 mcg of folic acid/Kg live weight (FA5) and the third group was dosed with 10 mcg of folic acid/Kg live weight (FA10). The results of the study revealed that both FA doses had significantly greater serum folates than the control group by the end of the fourth week of pregnancy (P<0.01). The FA10 group had a greater progesterone level than the FA5 and control groups at the fourth week of pregnancy (P<0.01) pregnancy. At the 2nd and 4th weeks of pregnancy, the FA10 group had greater levels of serum glucose and globulin than the control group (P=0.028 and 0.049, respectively). Both FA-supplemented groups had greater serum growth hormone (GH) levels at the 4th week of pregnancy (P=0.020). Additionally, the FA10 group showed significantly higher levels of IGF-1 at the 2nd and 4th week of gestation (P=0.040 and 0.001, respectively). FA supplementation decreased the levels of non-esterified fatty acid (NEFA) at the 2nd and 4th week of gestation (P=0.020 and 0.035, respectively). Additionally, the FA10 group showed significantly higher pregnancy associated glycoprotein (PAG) levels at the 2nd and 4th week of gestation (P=0.005 and 0.001, respectively). In conclusion, drenching of FA supplementation (10 mcg / kg) in the first month of gestation improved the blood folates, progesterone profile, as well as PAG, GH, and IGF-1 concentrations in heat-stressed Holstein cows. These findings could be useful in developing practical strategies to keep dairy cows’ regular reproductive patterns under heat stress conditions.