Each body tissue contains various proteins, and each of these proteins has a precise sequence of amino acids. Tissues also develop at different rates from birth to maturity. For example, the internal organs (i.e., liver, intestinal tract, and others) grow faster in younger pigs but more slowly in mature pigs. In contrast, other tissues (e.g., reproductive organs) develop more rapidly during the latter stages of growth. Because each tissue has a different amino-acid composition, the dietary amino-acid requirements will differ as these different tissues develop.
Muscle tissue develops at an increasing rate from birth until the mid to latter portion of the grower-finisher period, whereupon its rate of development declines. The period when muscle accretion declines largely depends upon the animal's genetic capability to deposit muscle. The potential number of muscle cells present at maturity is largely established genetically at conception, with their number completed by the time of birth. The leaner genotypes have a larger number of muscle cells at birth than those pigs that are not as lean. Muscle cells increase in size postnatally, but only in response to adequate nutrition and hormonal stimulation. Geneticists and seedstock companies have developed several improved lines, each having a specific rate of muscle development. The feeding of high-protein diets will not result in an increased muscle mass beyond the animal's genetic capability, but the feeding of a low-protein diet will clearly restrict its development and therefore its genetic potential. Consequently, it is possible to make a genetically lean pig fatter by feeding a low-protein diet, but impossible to make a genetically fatter pig leaner by feeding a high-protein diet. It is not possible to exceed the animal's genetic capability to produce muscle by nutritional means, but it is possible to restrict its genetic potential by poor nutrition.
The rate of muscle formation declines during the finisher period, but the timing and rate of this decline varies by the genetic makeup of the animal. There is currently a wide variation among genotypes, but most leaner genotypes have an increasing amount of muscle tissue to 160 to 180 lbs. before the rate of muscle development declines. In contrast, pigs with less muscle mass generally have a muscle deposition pattern that begins to decline at a lighter body weight (120 to 140 lbs.). Consequently, finisher pigs that have higher rates of muscle accretion will have a higher dietary amino-acid requirement than those genotypes with lower rates of muscle growth. Pigs of a lower muscle capability will therefore become progressively fatter as body weight is increased beyond 230-250 lbs.