A nutritional defense against stress
Cattle suffer various levels of stress depending on what stage of production they are in. Mitigating stress through management and feeding can pay dividends.
At weaning, calves experience a tremendous amount of stress and for a prolonged period. Proper nutrition can help alleviate some of the negative issues related to stress. It is important to understand how stress impacts the animal and how nutrition can help.
Animals subjected to stress have increased blood concentrations of the hormone cortisol. Cortisol can compromise the immune system. Specifically, cortisol will decrease T-lymphocyte cells, which are involved in cell-mediated immunity. Cell-mediated immunity is the portion of the immune system responsible for destruction of pathogens. Prolonged stress can reduce the animal’s ability to fight infection.
In order to mount an immune response, energy, protein and certain trace minerals are used, which increases nutrient requirements of the animal. Unfortunately, when calves are under stress, dietary intake is reduced. Research shows that fasting or feed deprivation can increase cortisol release. Therefore, the calf now has two problems: stress has compromised the immune system, and calves are not eating enough to meet the increased nutrient demands of an immune response.
Reduction in dietary intake is well documented in newly received feedlot calves. Upon arrival to the feedlot, intake can be as low as 60 percent of the consumption observed a week later. The National Academies of Sciences 2016 publication, “Nutrient Requirements for Beef Cattle,” suggests mineral concentrations in stressed calves be increased. Essential trace minerals hold an integral role in the immune system, with copper, zinc and selenium being of particular importance.
Copper is involved in the production of antioxidants and serves a role in neutrophil function, which is involved in the killing of pathogens. Although reports on the benefit of additional copper for improving immune function vary, the mere fact that it is necessary for proper function of the immune system warrants consideration when developing mineral programs. Studies show that when dietary copper supply and bodily reserves are low, there is a reduction in the ability of the immune system to kill pathogens. The publication states a non-stressed calf requires a diet that contains 10 parts per million copper. In a stressed calf, copper requirements increase up to 15 ppm.
Copper can be a challenging trace mineral to balance in a diet because of its relationship with iron, molybdenum and sulfur. If these minerals are too high, they can interact with copper and reduce its availability to the animal. For example, when total dietary sulfur intake increases above 0.2 percent, the amount of copper available for the animal decreases by approximately 25 percent. In some parts of the U.S., water sulfur concentration alone can be high enough to cause this reduction.
Zinc is involved in signals that initiate activity of certain cells in the immune system. Zinc requirements for non-stressed calves is 40 ppm and for stressed calves is up to 100 milligrams per kilogram. Some reports demonstrate a benefit from supplemental zinc, and some do not. In most cases, animals suffering from zinc deficiency exhibit positive responses to supplemental zinc. Therefore, calves not consuming adequate amounts of zinc are at a greater risk of becoming sick during periods of stress.
Selenium plays a role in immune function as it contributes to the production of antioxidants in the body and reduces oxidative tissue damage. Dietary selenium inclusion in diets of non-stressed calves is 0.1 ppm and up to 0.2 ppm for stressed calves. There are reports that demonstrate additional benefit of supplemental selenium even above 0.2 ppm. But federal regulations say that the selenium content of a complete feed cannot exceed 0.3 ppm or 3 milligrams per head per day. In animals fed diets deficient in selenium, selenium supplementation improves the immune system’s ability to combat pathogens.
Mineral supplementation programs generally focus on the cow; a good mineral program can positively impact cow and calf performance. In fact, calves generally consume mineral at the same time as the cow. However, their supplemental mineral consumption, as a percent of bodyweight, is approximately 50 percent of that of the cow. Thus, mineral status of calves as they go into the weaning season could be marginal. The associated stress could lower intake, which may exacerbate any mineral deficiencies and increase the risk of illness.
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