What are the prevention and control measures for heat stress in pigs during summer?
Heat stress refers to the sum of non-specific physiological responses made by an organism subjected to extremely high environmental temperatures in response to any demand the thermal environment places on it. With the rapid development of intensive high-density rearing methods, most southern regions increasingly and urgently need to alleviate the environmental pressure that summer high temperatures place on the @#@227@#@ industry, and the climate in the north also shows an upward trend; in recent years the highest temperature in Beijing has also reached over 40 degrees. The decline in pig productivity caused by heat stress has also increasingly drawn the attention of researchers in livestock environment and feed nutrition. Scholars from Japan, the United States, the United Kingdom and other countries began research in this field in the 1950s to 1970s of the twentieth century. Chinese scholars started late, entering this field in the early 1980s, seeking the regular changes that heat stress brings to animals, revealing the impact of the thermal environment on pig productivity, and developing regulatory means to alleviate the effects of heat stress.
I. Causes
(1) Pigs are homeothermic animals with thick subcutaneous fat, underdeveloped sweat glands, and relatively slow body heat dissipation, and they are not good at regulating body temperature through skin evaporation, so pigs tolerate heat poorly. Rearing practice has proven that the optimal temperature for pigs decreases as body weight and age increase: the suitable temperature for newborn piglets is 27°C to 29°C, for weaned piglets 21°C to 24°C, for growing-finishing pigs 15 to 25 degrees, and for farrowing and lactating sows 16°C to 18°C. Within the optimal environmental temperature range, heat production and heat dissipation in pigs maintain a dynamic balance, so the body temperature of pigs remains constant, feed utilization is high, growth is fast, and disease resistance is strong.
(2) When the environmental temperature exceeds the optimal temperature range, heat production in pigs exceeds heat dissipation, and pigs must regulate body temperature balance by increasing respiratory evaporation and radiant heat dissipation, or by reducing feed intake to reduce body heat production. When increasing heat dissipation and reducing heat production still cannot maintain the body's temperature balance, the pig's body temperature will rise. Foreign scholars have reported that piglets of 18 to 28 kg had their body temperature rise by 1.4 degrees above normal at 35°C, and Berkshire boars of about 60 kg body weight could not maintain normal body temperature when the environmental temperature was near 30°C. For pigs exposed to high temperatures for a long time, if excessive heat production accumulates in the body and cannot be dissipated, body temperature is likely to remain high, causing changes in the pig's neuroendocrine system and leading to heat stress or even heatstroke. The heat tolerance of pigs is related to breed and body size, body weight and economic type, among other factors. The smaller the body size, the more heat-tolerant; however, most breeds in China are now developing toward the three-way cross, with relatively fast growth rates, making them more prone to heat stress.
(3) With the scaling up and intensification of the pig farming industry, pig herd density has further increased, and the use of gestation stalls and the promotion of elevated farrowing rooms have made pig farms even more sensitive to high-temperature seasons.
(4) Many farm owners attach great importance to and are willing to invest in cold protection and warmth for pig farms in winter; however, they do not pay enough attention to summer high temperatures or invest enough in equipment upgrades.
II. Clinical manifestations and changes
(1) Manifestations. Pigs experiencing heat stress are depressed, with elevated body temperature, accelerated heartbeat, rapid breathing and even heat panting. They eat less, like to drink water, reduce activity, stretch out their bodies, and prefer to sleep on wet ground or roll in manure and urine. They avoid direct sunlight or move toward shady places, skin blood vessels dilate, and skin temperature rises. Growth slows or stalls, and they gradually become emaciated.
(2) Changes in the body. Boars show weakened libido, reduced semen volume, decreased sperm motility, and a decline in the proportion of live sperm to total sperm count. Gilts have delayed puberty and sexual maturity, sows have delayed estrus, silent estrus or even no estrus. Ovarian function and sexual function decline, and the conception rate drops significantly. When sows are mated at temperatures above 32°C, about 20% of pigs are infertile or return to estrus, and the abortion rate increases. High temperatures significantly reduce the number of surviving embryos within 8 days after mating; next, during the few days of embryo implantation (11 to 20 days after conception) they are particularly sensitive to heat, and after 20 days they have stronger heat resistance. Heat exposure in late pregnancy in sows can cause serious reproductive problems; in summer, the number of piglets born per litter and the number of live piglets decrease by 1 to 2, the sow's milk yield decreases, piglets grow poorly, the survival rate is low, and weaned piglets have low body weight.
(3) Effects of heat stress on pig growth
The optimal temperature range for growing-finishing pigs is 18 to 21°C, and 32 to 38°C is generally considered the heat stress range. Trials show that when the environmental temperature is 5 to 10°C above the optimum, pig feed intake decreases by 6% to 21%. Heat stress reduces pig feed intake and daily weight gain. The reasons can be explained as follows: (1) Heat stress alters the normal activity of the endocrine system in growing-finishing pigs, causing disorders in the secretion of hormones that regulate substance metabolism, such as thyroxine and adrenaline, thereby reducing the body's carbohydrate, lipid and protein metabolic activities, which in turn lowers the pig's daily weight gain rate. (2) High temperatures reduce the excitability of the feeding center, thereby causing a decline in feed intake. High temperatures cause a decline in feed intake, and reduced nutrient intake is the direct cause of reduced weight gain in pigs.
(4) Effects of heat stress on pig reproductive function
1. Sows: Under heat stress conditions, sows show reduced ovarian function, decreased conception rate, increased number of stillbirths at the end of gestation, reduced litter weight, and even abortion; boars show weakened mating desire and reduced semen quality. For boars, high temperatures make the proportion of boars with no mating desire higher than in normal-temperature seasons, and boars have reduced semen volume, a decreased ratio of live sperm to total sperm count, impaired spermatogenic function of the testes, an increase in multinucleated giant cells in semen as temperature rises, and a fertilization rate of 85% to 90% at 20°C, which becomes 50% to 60% after 72 hours of exposure at 33°C.
The mechanism by which heat stress affects pig reproductive function is explained as follows: heat stress alters the normal state of endocrine function, causing abnormal thyroid function, changes in thyrotropin secretion, and a significant increase in plasma adrenocorticotropic hormone; another explanation is that heat stress blocks the utilization of thyroxine, causing the body to exert feedback inhibition on thyroxine production. Although the mechanism affecting reproductive function is not yet very clear, it can at least be considered that after sustained heat stress, pigs will indirectly affect ovarian function, and in severe cases induce ovarian cysts.