A healthy animal give the best production results and a sick animal causes directly and indirectly unnecessary costs. The immune system accounts for a minor portion of total nutritional requirements in the healthy animal but in response to an immune challenge has a major impact on nutritional status and requirements for specific nutrients.
Nutrient deficiencies increase susceptibility to most infectious diseases and increase the probability of secondary infections. Vitamins or trace minerals deficiencies significantly depress immune function and resistance to stress even when animals are otherwise well fed with sufficient energy and protein. Nutrition has the largest impact on morbidity and mortality during the first month after arrival at the feedlot.
WHAT IS THE PROBLEM?
The immune system is suppressed in stressed cattle, contributing to the high incidence of respiratory disease in the first 30 to 45 days on feed. Lungs of healthy cattle can resist challenge from surprisingly large numbers of bacteria. But when the animal is stressed, relatively small numbers of bacteria can result in pneumonia. An additional aspect during this early stage is to adapt the rumen to easy digestible (starch) feed. If this process is not managed well, metabolic disturbances (acidosis) can easily occur. This causes a decline in intake, permanent damage to the rumen wall, dehydration and poor digestibility of ingested feed which imposes a huge challenge to the immunity of the animal.
Nutrition has the greatest impact on immunity during the first two to four weeks after arrival at the feedlot, even more so in the first few days after arrival. A problem is to get enough nutrients into cattle, particularly those that need it the most. Providing key nutrients can reduce stress-induced weight loss and immune suppression, improve weight gain and reduce morbidity and mortality from bovine respiratory disease.
WHAT IS IMMUNITY TO INFECTION?
Animals resist infection using non-specific mechanisms (‘innate’ immunity) and specific mechanisms (acquired immunity). Innate immunity includes epithelial tissue which covers body surfaces (example skin or hide, hoof, cornea) and lines body cavities (respiratory, urogenital, gastro-intestinal) is a physical barrier to keep infectious agents from entering the body. Acquired immunity is where exposure to a foreign substance (antigen) results in the development of immune cells and antibodies specific against that particular antigen only. Specific immunity takes time to develop, and therefore is effective in preventing infection only if the animal was previously exposed to that antigen. A nutrient deficiency does not affect all immune mechanisms equally, nor does it always affect the same mechanism equally for different antigens. A group of deficient animals may have adequate antibody responses to some antigens (or vaccines), and depressed antibody responses to other antigens. Similarly, under practical conditions, deficient animals may be more susceptible to some but not all disease causing organisms. That is why the symptoms of disease are presented only with a percentage of animals although all the animals have been exposed to the infection.
WHAT IS THE NUTRITIONAL REQUIREMENTS OF STRESSED ANIMALS?
Acute (immediate) or prolonged (two to seven days) stress makes animals more susceptible to disease. Stressful stimuli in cattle include handling, transport, physical trauma, fatigue, fasting, and unfamiliar environment. Increasing levels of hormones like cortisol, epinephrine, norepinephrine, aldosterone, beta-endorphin, and encephalin are released during stress. These induce hormonal responses which control and alter animal metabolism hormones, suppress the immune system by depleting nutrients critical to an effective immune response. Metabolic pathways shift from anabolic processes (growth) to primarily catabolic (tissue breakdown of proteins and fat).
WHAT IS THE EFFECTS OF DISEASE ON NUTRITIONAL STATUS?
Infection results in a complex array of metabolic responses which affect the nutritional status of the animal. Feed intake decreases more than 50% in cattle with respiratory disease and fever, and takes 10 to 14 days to return to normal. During this time, requirements of virtually all nutrients increase, even though nutrients are deflected from growth to immunity. Proteins must be synthesized for the immune response, development and resolution of fever, and repair of cell and tissue damage. Body proteins are broken down to provide energy and amino acids for the immune system. Losses of important minerals such as magnesium, potassium, phosphate, and zinc are also increased. The major extracellular electrolytes, sodium and chloride, are influenced by hormonal changes. Urinary excretion of salt may increase during the onset of infection, and may lead to dehydration. The kidneys may then begin to retain body salt and water. Diarrhoea can result in substantial direct faecal losses of sodium, chloride, bicarbonate and potassium. The immune system appears to have priority for nutrients over growth. Immune activation is costly. Resources devoted to immune activation cannot be used for growth. As a result, any immune response will depress growth rate and feed efficiency. An extreme immune response not only wastes resources but may cause local cell damage and depress productivity.
WHAT IS THE EFFECT OF NUTRITIONAL STATUS ON RESISTANCE TO INFECTION?
Good nutrition improves disease resistance of stressed cattle, by helping to counteract the suppression of the immune system caused by stress hormones and by providing nutrients essential for maintaining and activating the immune system as required. Generally, nutrition has most impact on health during the first month on feed and particularly for highly-stressed or lighter weight cattle. Many of the cattle entering feedlots are marginally deficient in several trace minerals, especially if they previously grazed dormant native range or pasture. Receiving rations should be adequately supplemented with trace minerals, vitamin A and vitamin E. Feeding higher levels than normal of these nutrients in the receiving period can compensate for reduced intake and marginal deficiencies on arrival. After cattle have recovered from stress induced immune suppression and are adapted to the finishing diet, balancing feedlot diets to meet requirements for growth usually provides adequate nutrients for immune function. In order of priorities, providing a palatable diet on arrival and encouraging feed and water consumption is most important. There is little benefit from adding expensive nutrients to an unpalatable diet. Research has shown that it can take up to a week or more for all calves in a pen to consistently consume some feed within a single day. Average intake is only 1.5% of body weight in the first 2 weeks for lightweight feeder calves. Anything that can be done to enhance intake after arrival reduces the length of time in which the immune system is compromised by short-term nutrient deficiencies.
WHAT IS OPTIMAL IMMUNITY?
The optimum immune response is the one that is the shortest in duration and the lowest in intensity while successfully eliminating the pathogen. Reducing challenges to the immune system by providing a clean environment and reducing stress can improve growth rate, even in the absence of disease. In normal healthy cattle, there is a trade-off between growth and immunity – both cannot be maximized at the same time. This is not usually a practical concern because the immune system functions very well at nutrient levels ideal for growth. Compared to the adapted feedlot steer, the stressed feeder calf is in a unique situation because of the conflicting nutrient demands for immune activation (vaccination and/or disease exposure), responding to stress, recovering from short-term nutrient deficiencies, and growth. This animal has potential to respond to some key immune enhancing nutrients with significantly increased gain and improved immunity.