Causes and Preventive Measures of Pig Stress

Pig stress is an unavoidable problem in the @#@227@#@ production process. The harm caused by stress can be either single or comprehensive, and its effects are multifaceted. If proper preventive measures can be taken according to different specific situations, the losses caused by stress will be greatly reduced.

Stress is the sum of nonspecific response reactions produced by the organism to various extraordinary stimuli. General stimuli cause specific reactions in the organism. However, some stimuli not only cause specific reactions in the body but also trigger nonspecific reactions, which are manifested as:

① Thickening of the adrenal cortex and increased secretory activity;

② Atrophy of the thymus, spleen and other lymphoid tissues, decreased eosinophils and lymphocytes in the blood, and increased neutrophils;

③ Ulceration and bleeding of the stomach and duodenum. These symptoms are called "General Adaptation Syndrome (GAS)." Therefore, stress is not an environmental stimulus but a response of the organism.

I. The occurrence and development of stress can be divided into three stages under typical conditions.

1. Alarm reaction and mobilization stage: GAS reactions appear, catabolism predominates, pigs become emaciated, negative nitrogen-carbon balance occurs, and the body's resistance falls below normal levels. This stage generally lasts 6-48 h. If the stress factor is extremely strong, pigs may die within 1 h or 1 d; if the organism overcomes the influence of the stress factor and survives, it then enters the adaptation stage.

2. Adaptation or resistance stage: The organism has adapted, metabolism returns to normal, anabolism predominates, physical condition is restored, the body's resistance rises above normal levels, and resistance to other stimuli is also increased. This stage generally lasts several hours, days or weeks. If the stressor stops acting or its effect weakens, and the organism overcomes the adverse effects of the stimulus, stress ends at this stage. In most cases, stress only goes through the above two stages, but if the stressor continues to act or the organism cannot overcome this effect, it enters the exhaustion stage.

3. Exhaustion stage: This stage resembles the alarm stage, but the degree of reaction increases sharply. Although the adrenal cortex is hypertrophied, it cannot produce the necessary hormones; tissue proteins, fats and other body reserves are decomposed, and body weight drops sharply; the acquired resistance is lost again, adaptive functions are destroyed, the functions of various systems become disordered, and finally exhaustion leads to death.

II. Stressors and Their Effects

1. Stressors

Stress can cause pigs to produce nonspecific responses and nonspecific changes, namely General Adaptation Syndrome (GAS). Any stimulus that can cause the organism to develop GAS is called a stressor. It is conventionally divided into:

① Feeding and management factors: including confinement, dense stocking, catching, regrouping and transport, fighting, malnutrition, immunization, castration, ear tagging, tail docking, etc.;

② Environmental factors: including extreme heat, severe cold, strong radiation, low air pressure, toxic and harmful gases, dust, moisture, strong winds and drafts, noise, etc.;

③ Microbial infection factors: bacteria, viruses, parasites, mycoplasma and chlamydia, etc.;

④ Other human factors: such as intensive exploitation of production performance, discomfort with machinery and equipment, etc.;

⑤ Genetic factors: pigs homozygous for the recessive halothane gene are prone to stress syndrome.

2. Harm of stress

(1) Reduced pork quality

① PSE meat, which is pale in color, soft in texture and exudative in moisture, is mainly caused by crowding, binding, and cold and heat stimulation during transport; DFD meat, which is dry, coarse and firm in texture and dark in color, is mainly caused by long-term stimulation by low-intensity stressors.

② Substantive myocarditis (acute heart failure) occurs mostly in pigs aged 4-6 months, characterized by sudden death without warning.

③ Necropsy reveals pale, grayish-white or yellowish-white myocardium with striated or spotted degeneration.

④ Necrosis of the longissimus dorsi muscle in adult pigs, with necrosis of the back muscle visible after slaughter.

⑤ Acute necrosis of the leg muscle in pigs, with the leg muscle showing acute serous necrotic inflammation, mainly caused by binding and squeezing during long-distance transport.

(2) Low immunity

Under stress, the thymus, spleen and lymphoid tissues atrophy, the production, differentiation and activity of eosinophils, T lymphocytes and B lymphocytes are hindered, blood phagocytic activity is weakened, and antibody levels in the body are low, thereby inhibiting cellular immunity and humoral immunity, leading to decreased immunity and weakened disease resistance. Bacteria such as Escherichia coli and Pasteurella multiply rapidly, their virulence increases, and they invade the blood, causing gastric ulcers, bacteremia, sudden death syndrome, etc. in pigs.

(3) Reduced production performance

During stress, the body must mobilize large amounts of energy to cope with the stimulation of the stressor, which enhances the catabolism of body proteins, carbohydrates and fats, reduces anabolism, and increases glucocorticoid secretion, leading to growth arrest, weight loss and reduced feed conversion rate, manifested as severe weight loss in livestock during transport and while waiting for slaughter.

(4) Reduced reproductive capacity

Stress can reduce the secretion of follicle-stimulating hormone (FSH), luteotropic hormone (LTH), etc.; cause gonadal hypoplasia in young pigs, gonadal atrophy, reduced libido, and poor sperm and egg development in adult pigs; and can affect fertilized egg implantation and fetal development, causing early resorption, abortion, fetal malformation or stillbirth.

(5) Under high-density rearing conditions, it can trigger cannibalistic behavior among animals of the same species.

(6) During long-distance transport, pigs are prone to acute mycoplasma pneumonia, sunstroke, heatstroke, etc., manifested as depression, elevated body temperature, accelerated breathing, cyanotic mucous membranes, muscle tremors, foaming at the mouth or vomiting, leading to death.

III. Measures to Reduce Stress Effects

1. Select stress-resistant pig breeds. Different pigs have different sensitivity to stress. When purchasing and introducing piglets, attention should be paid to selecting breeds with strong stress resistance.

2. The pig house structure should be scientific and reasonable, and the micro-environmental conditions inside the house should be improved.

3. According to the different growth stages of pigs, scientifically allocate diets. The nutritional level of the feed should meet the needs of the animals, and feeding should be carried out at fixed times and in fixed amounts. Do not feed moldy and deteriorated feed. Drinking water should be clean and disinfected. Troughs and water troughs should be sufficient. Attention should be paid to hygiene to avoid fighting over food and uneven feed intake. At the same time, the following work can be done well:

(1) Appropriate amounts of carbohydrates and fats

Adding 2% vegetable oil to the diet of growing pigs and correspondingly reducing the carbohydrate content can reduce body heat production in pigs, lighten the heat dissipation burden of pigs, and alleviate high-temperature stress.

(2) Reasonable protein and amino acids

It has been reported that balancing amino acids and reducing crude protein intake are important measures to alleviate heat stress in pigs. Feeding lysine to replace natural protein is beneficial to pigs because lysine can reduce the heat increment of the diet. Under hot climate conditions, if ideal protein is used as the basis and the lysine content in the diet is increased, feed conversion rate can be improved, and there is no significant difference in pig production performance and carcass quality compared with conventional diets.

(3) Adding vitamins

In hot weather, adding 100 g of stress hormone to every 150 kg of feed helps reduce the impact of heat stress on sperm quality and fertilization rate; it can regulate substance metabolism in the pig body, enhance immune function, improve stress resistance, reduce body temperature and respiratory rate of market pigs under heat stress, and effectively improve the production performance of finishing pigs; it can also effectively prevent diarrhea caused by vitamin E deficiency.

(4) Use of trace elements

① Chromium supplementation has certain effects on resisting stress, improving production performance, regulating endocrine function, influencing immune response and improving carcass quality.

② Copper has antimicrobial properties, and copper combined with antibacterial agents can produce a synergistic effect.

③ Adding arsenic preparations to piglet diets can effectively control diarrhea and increase weight gain.

④ Selenium is a component of glutathione peroxidase (GSH-PX) in livestock and poultry. Through this enzyme, peroxides are converted into harmless alcohols to prevent unsaturated fatty acids in cell lipid membranes from being damaged by peroxides. Adding organic selenium has positive effects.

(5) Drug prevention and treatment of stress

In order to improve the body's stress resistance and prevent and treat stress, anti-stress drugs can be given through feed, drinking water or other routes. Anti-stress drugs are currently the most active field in anti-stress research and have achieved considerable development.

(6) Prevent stress during transport and slaughter

It is best to fast the pigs before transport, and 100 g of stress hormone can be added to 300 kg of drinking water. This can prevent the effects of unfavorable factors such as crowding, sun exposure, wind and rain during transport. The length of time waiting for slaughter should be appropriate.