In addition to these innate contributors to variability, there are many external forces that come into play as well, such as health status, access to resources (feed, water) and poor ventilation.

• Prenatal influences: Variation begins on the day pigs are born. Studies of neonatal and weaned pig management have reported that even within a litter, the CV for birth weight is between 22% and 26%. As litter size increases, the average birth weight declines by about 0.1 lb. for every additional piglet in the litter.

  While differences in birth weight are obvious to any farrowing technician, what is less clear is the relationship between birth weight and physiological “competency” at birth.

  For example, differences associated with low birth weight have been observed in reduced number and height of intestinal villi, lactase and lipase activity, reduced muscle respiratory enzyme activity, fewer muscle thyroid hormone receptors, lower IGF-1 levels in the blood and fewer muscle fibers.

• Postnatal influences: After birth, additional factors contribute to variability. For example, heavier birth weight piglets consume about 30% more milk than their lighter littermates. In addition, heavier birth weight piglets, or at least those that win the most fights early in life, tend to suckle the anterior teats on the sow, which are known to deliver higher milk volumes. In one study, pigs nursing the anterior teats were 3.3 lb. heavier at weaning than those adopting the posterior teats.

  Lower milk intake is not only associated with slower growth, but also reduced whole-body protein synthesis, according to research conducted in the United Kingdom. That research also reported that tripling milk intake during the first week of life quadrupled the pigs' protein deposition rate.

• Postweaning influences: One of the most predictable contributors to variability in the postweaning period is the variability in weaning weight. For example, the correlation between weaning weight and nursery exit weight was found to be 0.73. Numerous authors have related weaning weight to nursery exit weight by suggesting that for every 1 lb. increase in weaning weight, nursery exit weights will increase by a given amount.

  At the Prairie Swine Center Elstow Research Farm, we have found that for every 1 lb. increase in weaning weight, there is a 1.9 lb. increase in nursery exit weight (56 days of age) and a 4.2 lb increase in market weight. This relationship varies widely among farms.

• Herd health and pathogen exposure: While inherent factors such as birth weight, weaning weight and suckling habits affect variability in pigs, there are many other “external” forces at play as well.

  Perhaps the most important external force on variability is the disease status of the herd. Because the extent of disease exposure differs among animals, and because the impact of that exposure on animal health and performance also differs among individuals, it is not surprising to observe that herd health status can have a major impact on variability.

• Feed and water: Access to resources, such as feed and water, is also a potential contributor to variability.

  If feed or water access is limiting, dominant pigs in a pen will have an advantage over subordinate pigs, which results in greater growth disparity.

Social Behavior Model

There is a very interesting social behavior model for describing variability where one considers the average performance of a group of pigs along with the magnitude of the variation about this mean.

If average performance is very good and the degree of variation is low, one can assume that the conditions in which the pig are being reared are good and there is no depression in growth due to aggressive behaviors. Conversely, where performance is suboptimal and variation is high, one can assume there is competition for resources, such as food and water.

Overcrowding is not a likely issue in this scenario, because space is not a resource that pigs can hoard away from other pigs in the pen. However, pigs can prevent others from eating or drinking. The resultant uneven distribution of feed and water does lead to increased variability.