Heat stress and its control in pigs in summer

Heat stress refers to the sum of non-specific physiological responses of the body to any requirements put forward by the thermal environment in an extremely high ambient temperature. With the rapid development of intensive and high-density feeding, most southern regions are becoming more and more urgent to alleviate the environmental pressure caused by high temperatures in summer, and the climate in the north is also increasing. In recent years, the highest temperature in Beijing has also reached more than 40 degrees. The decline of pig productivity caused by heat stress has attracted more and more attention of livestock environment and feed nutrition researchers. Scholars from Japan, the United States, Britain and other countries have carried out research in this field in the 1950s and 1970s. Chinese scholars started late and have been involved in this field since the early 1980s, seeking the regular changes brought by heat stress to animals, revealing the effects of thermal environment on pig productivity, and developing regulation and control means to alleviate the effects of heat stress.

I. the cause of occurrence

(1) Pigs are thermostatic animals with thick subcutaneous fat, underdeveloped sweat glands, slow heat emission in the body, and are not good at regulating body temperature through skin evaporation, so pigs are not heat-resistant. The feeding practice showed that the optimum temperature of pigs decreased with the increase of body weight and age. the optimum temperatures of newborn piglets, weaned piglets, growing and finishing pigs were 27 ℃ ~ 29 ℃, 21 ℃ ~ 24 ℃, 15 ~ 25 degrees and 16 ℃ ~ 18 ℃ respectively. In the range of optimum ambient temperature, the heat production and heat dissipation of pigs maintain dynamic balance, so the body temperature of pigs is constant, the feed utilization rate is high, the growth rate is fast, and the disease resistance is strong.

(2) when the ambient temperature exceeds the optimal temperature range, the heat production of pigs is greater than that of heat dissipation, so pigs have to adjust their body temperature balance by increasing respiratory evaporation and radiation heat dissipation, or by reducing feed intake to reduce body heat production. When the body temperature balance can not be maintained by increasing heat dissipation and reducing heat production, it will cause a rise in pig body temperature. Foreign scholars reported that the body temperature of 18-28 kg piglets increased by 1.4 degrees from the normal body temperature at 35 ℃, and the Baxia boar weighing about 60 kg could not maintain normal body temperature when the ambient temperature was nearly 30 ℃. For pigs exposed to high temperature for a long time, if too much heat accumulation can not be dispersed in the body, it is easy to keep the body temperature high, resulting in changes in the neuroendocrine system of pigs, heat stress and even heat radiation disease. The heat tolerance of pigs is related to breed, body size, body weight and economic type. The smaller the body is, the more heat-resistant it is, but now most of the varieties in China are developing outward, growing faster and more prone to heat stress.

(3) with the large-scale intensification of the pig industry, the density of pig herds has further increased, and the use of tire protectors and the promotion of high-bed delivery rooms have made pig farms more sensitive to high temperature seasons.

(4) many ranchers pay more attention to the cold protection and warmth of pig farms in winter, but they do not pay enough attention to the high temperature in summer and the investment in equipment transformation.

II. Clinical manifestations and changes

(1) performance. The pigs with heat stress reaction had depressed spirit, elevated body temperature, rapid heartbeat, shortness of breath and even febrile wheezing. Reduce food intake, like drinking water, reduce activity, stretch the body, like sleeping in wetlands or lying in feces and urine. Avoid direct sunlight or tend to a cool place, the skin blood vessels dilate, skin temperature increases. Slow or stagnant growth, wasting away day by day.

(2) changes in the body. The libido of boars decreased, the ejaculation volume decreased, the sperm motility decreased, and the ratio of live sperm to total sperm decreased. Young sows delayed proestrus and sexual maturity, delayed estrus, recessive estrus and even no estrus. The ovarian function and sexual function decreased, and the conception rate decreased significantly. When the temperature reached more than 32 ℃, about 20% of the pigs were infertile or repeated estrus, and the abortion rate increased. High temperature significantly reduced the number of embryos survived within 8 days after mating, especially sensitive to heat within a few days of embryo implantation (11-20 days after conception), and had strong heat resistance after 20 days. Serious reproductive problems can occur when sows are heated in the late pregnancy. In summer, the litter size and live litter size of sows are reduced by 1 to 2, the lactation of sows is decreased, the growth of suckling pigs is poor, the survival rate is low, and the body weight of weaned piglets is small.

(3) effects of heat stress on the growth of pigs

The most suitable temperature range for growing and finishing pigs is 18-21 ℃, and 32-38 ℃ is generally considered to be the range of heat stress. The results showed that when the ambient temperature was higher than 5: 10 ℃ of the optimum temperature, the feed intake of pigs was reduced by 6% and 21%. No, no, no. Heat stress decreased feed intake and daily gain of pigs. The reasons can be explained as follows: (1) Heat stress changes the normal activity secreted by growing and fattening pigs, which leads to the disorder of hormones that regulate the metabolism of substances such as thyroxine and epinephrine. as a result, the metabolic activities of sugars, lipids and proteins are reduced, and then the daily weight gain rate of pigs is reduced. (2) High temperature decreased the excitability of the feeding center, resulting in a decrease in feed intake. The decrease of feed intake caused by high temperature and the decrease of nutrient intake is the direct reason for the decrease of pig weight gain.

(4) effect of heat stress on reproductive function of pigs.

1. Sows: under the condition of heat stress, sows showed decreased ovarian function, decreased conception rate, increased number of stillbirths at the end of pregnancy, decreased litter weight, and even abortion; boar mating desire decreased, semen quality decreased. For boars, high temperature made the proportion of pigs without mating desire higher than that in normal temperature season, and the ejaculation volume of boars decreased, the ratio between live sperm and total sperm decreased, the spermatogenic function of testis was impaired, and the number of multinucleated giant cells in semen increased with the increase of temperature. The fertilization rate was 85% 90% at 20 ℃ and 50% 60% after 72 hours exposure at 33 ℃.

The mechanism of the effect of heat stress on the reproductive function of pigs is explained as follows: heat stress changes the normal state of endocrine function, abnormal thyroid function, changes in the secretion of thyrotropin, and a significant increase of adrenocorticotropin in plasma. there is another explanation: heat stress hinders the utilization of thyroxine and causes feedback inhibition of thyroxine production. Although the mechanism affecting reproductive function is not very clear, it can at least be considered that continuous heat stress in pigs will indirectly affect ovarian function and, in severe cases, induce ovarian cysts.

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