Dealing with Variation in Pig Barns
By Mark Bertram, Ph.D., First Choice Livestock
Variation is a fact of life in pig barns. There are many reasons for it – some may be rooted in the impact that fetal imprinting has on muscle fiber numbers and the genetic potential for growth that any animal possesses. Whatever the real reasons, your job is to understand the impact that it has on feeding strategies and barn closeouts so you can become more profitable and use facilities more efficiently.
Research has shown that we can predict with good certainty the number of pigs that will fall into given weight categories. Statistically we look at variation this way: 99% of all pigs are within three standard deviations of the mean, 90% are within two standard deviations, and 66.7% are within one standard deviation. The typical standard deviations for pigs at different stages of growth are: weaning - 2.25 lb, end of nursery stage - 11 lbs; end of finishing period - 24 lb.
So if your average weaning weight is 11 lbs, two thirds of your weaned pigs will be between 8.75 lbs and 13.25 lbs. If these same pigs average 50 lbs going into the grower barn, two thirds will be between 39 lbs and 61 lbs. And at the end of finishing -- when weight becomes the most important – two thirds will be in a 48-pound range around your target shipping weight.
Unfortunately, weaning weight is not a good predictor of ending weight – it only explains about 25% of the variation in ending body weight. Thus, sorting and segregating pigs based on weaning weight is not effective in reducing ending weight variation. In fact, when you look at light pigs at weaning (more than one standard deviation less than the mean weight), only 35% of them ended the finishing period with weights that were more than one standard deviation less than the mean and 24% ended with weights that were more than one standard deviation greater than the mean. So we have learned that the standard deviation of ending weight is not influenced by the level of performance. The curve is shifted left or right based on the level of performance of a given group, but the range is not influenced.
So, how does this variation influence how we feed pigs and market the barns to maximize pounds of pork per square foot of barn space? Diets should be targeted to the nutrient requirement of pigs that are one standard deviation lighter than the mean of the barn. This will result in approximately 83% of the pigs being at or above the nutrient level that is required for their rate and composition of growth. Additionally, since the lightest 17% have growth rates that are less than the average, their nutrient requirements will nearly be met with the same diet.
On the marketing side, you should examine the strategy that best meets your marketing agreement, however if the packer matrix is one of the many that allow carcasses that are +/- 20 to 30 pounds around a preferred mean a strategy that takes 2 cuts and a dump works very well. Cuts of 30%, 30% and 40% provided optimal balance between packer sort and discount matrices and marketing maximum pounds of pork per square foot of facility -- which should be the goal of any operation.
While it would be nice to wave a wand and decrease the weight spread of a wean-to-finish barn, the realities of variation dictate strategies that benefit the best doing pigs to the same extent as poorer doing pen mates. If you understand the magnitude of this variation, feed the right diets, and market the right amount of pigs at the right time, you stand the greatest chance of making money.
Mark Bertram is a Swine Nutritionist from Polk City, Iowa. He received his Ph.D. from Iowa State University and has been in the feed and swine production industry for 12 years. His main focuses are nutritional program design and implementation, feed quality control and production management. Mark can be reached by phone at (515) 290-2979 or email at mbertram@firstchoicelivestock.com.
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