When expressed as a percentage of the diet, most nutrient requirements change with age or maturity (Figure 5). Although this change occurs daily, under practical conditions it is not feasible or necessary to reformulate the pig's diet that frequently. Feeding several diets (phase feeding) during the grower-finisher period will lower feed costs, but there may not be much difference in the rate of gain. One of the more important considerations in the phase feeding of pigs from 50 lbs. to market weight is that excess nutrients are not being fed when the pig does not need them, and therefore there is less wastage of nutrients in the excrement. The number of diets used in a grower-finisher feeding program will usually range between three and six.
Some have advocated the withdrawal of die-tary vitamins and minerals during the last four to six weeks of the finisher period. This practice could affect the nutrient content of the meat, which is one of the desirable attributes of pork. It should be recognized that when pigs are growing, they have a metabolic as well as an immunological need for vitamins and minerals. If the pig is challenged by an infectious disease, the immune system may not function optimally if the body becomes deprived of critical nutrients. Although body tissues can store some vitamins and minerals for a short time period, they will divert nutrients from body stores to meet their metabolic needs, thus depleting body tissue reserves. Because nutrient withdrawal from the diet may affect the nutritional value and shelf life of pork, the practice is not recommended.