For livestock growing in temperate regions, heat stress adversely affects most of the factors associated with reproductive success, including both female and male animals.
Current limited research suggests that heat stress delays puberty in both female and male ordinary cattle. 10 Juanshan bulls aged 26 weeks were tested and tested for 8 hours a day at 35 °C, and their puberty was delayed compared with similar bulls that were not stressed.
Short-horned heifers raised at 27 °C are 2 months later than similar heifer puberty at 10 °C. In the same study, heifers hybridized by ordinary bovine × tumor cattle were not affected by temperature changes.
It is speculated that the delay in puberty is due to the decrease in appetite caused by heat stress, inhibition of thyroid activity, and thus slower growth.
Short-horned heifers raised between 27 °C or 10 °C enter puberty at a similar period of shoulder blade height, but the weight is different, and the main problem after bull puberty is sperm quality. For cows, all aspects of reproduction are affected; however, the most significant effects are estrus performance and embryo survival.
01
<b>Sperm quality</b>
In most parts of the world, the effects of summer have been severe enough to reduce sperm quality.
For ordinary bulls, an ambient temperature above 27 ° C is enough to reduce sperm quality as long as it reaches 6 h per day. Thermal stress effects include decreased sperm concentration and forward motility, and an increase in sperm count in abnormal morphology.
Decreased sperm quality can reduce fertility. Placing a Gernsey bull at a temperature of 29.4°C or higher for 5 weeks or more can weaken sperm production. Similar findings have been made in Holstein bulls and Swiss brown cattle, with less influence on the hybrid offspring of Red Cow × Holstein bull or Red Bull × Swiss Brown Bull (Table 1).
After 9 weeks of heat stress, sperm quality recovered significantly, and hybrid bulls recovered faster and more thoroughly.
Many recent studies have revealed that in the sperm extracted from the body of the hot stress bull, the binding force of mucopolysaccharides and liver phospholipids to sperm decreases. This suggests that thermal stress may alter the integrity of the sperm membrane. Mucopolysaccharides are a group of unbranched linear polysaccharides that enhance the apical (sperm) response. Moderate heat stress at fertilization rates leads to increased early embryo mortality.
Table 1
The failure of pregnancy due to low sperm quality due to heat stress can be solved by the following measures, that is, the collection of high-quality semen from male animals in the heat-neutral environment and artificial insemination. This method is particularly suitable for cattle, as frozen bovine semen is available worldwide and the price is acceptable.
However, investigative evidence suggests that the entry of high-quality semen into heat-stressed male animals (1 to 2°C above normal) may alter sperm and reduce embryonic survival.
For example, the cultivation of rabbit sperm at 40 °C for 3 h did not change the fertilization rate compared to 38 °C, but it did adversely affect the survival rate of the embryo.
Thermal imager
02
<b>Estrus expression</b>
Reducing the intensity of estrus is the highest limiting factor controlling the breeding of ordinary cows in the summer. This translates into reduced creeping vitality and shortened estrus period.
In a study in Virginia, the number of Holstein cattle in the summer period was less than half that in the winter. The institute in Louisiana, located in the southern center of the United States, reported that the average estrus period of Holstein and Juanshan cattle was 11.9h, which was 5h shorter than in areas where cooler areas had been reported.
In the follow-up study, the estrus period of cows raised under cool conditions was 20h, while the heat stress cows raised indoors in artificial environments and in natural environments in summer were 11h and 14h respectively.
The problem of hot climates affecting more is reported to be lackluster, but in most cases, these studies did not distinguish between true analgesic periods (acyclic) and estrus misjudgments.
In a study in Pakistan, the maintenance of estrus cycles during the summer months (July to October) with Holstein and Juanshan cattle as participants was manifested by progesterone secretion cycle patterns (Figure 1).
However, only 35% to 40% of cattle in the cycle observed estrus, although the study start time and estrus observation time were synchronized, every 6 h. Therefore, it seems that the main problem is a weakening of the intensity of estrus expression, which makes detection difficult, or may lead to calm ovulation.
Figure 1
Because the intensity of estrus expression is weak, more work needs to be done to detect estrus in the summer. In subtropical regions, the detection efficiency is increased by 20% every 6 hours compared with the detection every 12 hours.
The use of crawl detectors further improves efficiency. Proven effective crawl detectors include the tail paint method, the commercial crawl detector, and, most recently, a radiometric pressure converter capable of conducting the number and time of crawls, which can be placed at the base of the cow's tail when used.
In summer, artificial cooling methods for cows include the use of shades and fans with or without sprinkler systems, which promote estrus expression.
A Mississippi study of lactation-stage Holstein cattle showed estrus in 71 percent of cases where they were cool and used a fan while injecting two simultaneous injections of prostaglandin F2α, compared with 33 percent in the case of cool fansless.
In an Israeli study, intermittent sprinklers and fans cooled Holstein cattle short-term, and more cows showed routine estrus (70%) compared to unscaled cows (45%). Ovulation without estrus is more likely to occur in unheated cows.
Thermography – the surface temperature of a cow
Because summer cows are in estrus cycles, controlled breeding methods, including simultaneous injections of specific hormones, should help detect estrus and lead to an increased probability of pregnancy.
In particular, synchronized artificial insemination that accompanies ovulation can improve fertilization rates because, in essence, all estrus cows can undergo artificial insemination near the ovulation period. If this is not the case, fertilization can only be performed when estrus is detected.
Although the percentage of fertilized animal pregnancies does not increase, a large number of fertilized animals also results in a higher number of pregnancies.
In a Florida study, Holstein cattle undergoing timed artificial insemination with synchronized ovulation had a higher pregnancy rate (16.6%) in the 90th postpartum period than Holstein cattle in the control group (9.8%). In another study, the pregnancy rate of timed artificial insemination with synchronized ovulation within 90 days was 34.3%, compared with 14.3% for cows in the control group.
Public number: Harold Technology
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