Pork producers look with envy at the broiler industry, which empties its barns in a single day, while we take 3 to 5 weeks to accomplish the same end. In the broader scheme of things, variability is both a curse and a blessing to our industry. That being the case, management of variability becomes an essential key to success in pork production.

Because variation is the foundation of genetic selection, the industry’s ability to select breeding stock for certain desirable traits in performance, meat quality, carcass quality, reproductive performance or even disease resistance requires variation within the pig population. In this regard, variability is a good thing, and indeed, an essential part of our industry. Such variation can be managed, at least to some extent, by utilizing synthetic lines or crosses of pigs to achieve a more uniform final market animal. Clearly, we must be careful to not allow our desire for uniformity to diminish the availability of diverse breeds or lines within breeds that are the essence of future selection programs.

While genetic variation is a fundamental characteristic of the pig population, the environment clearly plays a significant role as well. This is perhaps best exemplified in cloning experiments. For example, many people were shocked when the famous Canadian Holstein dairy bull, Starbuck, was recently cloned at the University of Montreal and his son exhibited quite a different skin coat pattern. This was clearly unexpected when the genetic composition of Starbuck II was identical to that of his progenitor; how profound, then, is the impact of the environment on the expression of genetic make-up? If the environment has such an important influence on livestock performance, what strategies can pork producers adopt in order to most effectively and efficiently deal with variability?

What is variability

First, what is meant by variability, and how much variability typically exists within a herd. Variability is defined statistically in many ways. One common term is “standard deviation.” Standard deviation is a measure of the amount of variation that exists about the average of a group of pigs. It is defined such that one standard deviation about the mean will include about 66% of all pigs within that group; two standard deviations will include 95% of the pigs, and three standard deviations will encompass more than 99% of all of the pigs. For example, if a pork producer recorded the weight of all of his pigs at 140 days of age (20 weeks), the average might be 95 kg and the standard deviation 11 kg. In this example, 2/3 of the pigs would weigh between 84 kg (95 – 11) and 106 kg (95 +11). More than 95% of the pigs would weigh between 73 kg (95 – 11- 11) and 117 kg (95 + 11+ 11). When one talks about reducing variability, one is really talking about reducing the standard deviation. Figure 1 helps to explain the concept of standard deviation in graphic terms.

Sometimes, people prefer to use the term “coefficient of variation” or CV; it is the standard deviation expressed as a percent of the average. In the above example, the CV would be 11.6% ([11/95] x 100). In typical pig populations, the CV is commonly in the range of 8 to 12%.

Two fundamental approaches exist. The first would be to manage the environment in such a way as to minimize variability, and the second would be to manage the pig to minimize the impact of variability on net income. In other words, one approach would be to seek ways to minimize variability, and the second would be to manage whatever variability exists within a herd.

Minimizing the Amount of Variability

In recent years, there have been many attempts at reducing the amount of variability in a group of pigs. Most have failed. For example, until recently, it was common practice to sort pigs as they enter the nursery or the growout barn, to achieve greater uniformity with the pen, and thus increase the uniformity of growth. Recent studies completed at the Prairie Swine Centre found that sorting failed to reduce variability, and if anything, had an adverse impact on overall performance; these findings were later confirmed by experiments at Kansas State University.

Other research at the Centre has found that only severe restriction of access to feed will increase variability. Changing group size or crowding pigs was also found to have no effect. As shown in Table 1, the coefficient of variation of bodyweight was unaffected by providing additional floor space per pig. We have also looked at the impact of nutritional status of the diet, and found that only when amino acids, for example, are severely limiting is variability increased.

We expect to conduct additional research in this important area, and indeed, expect to see new results from other institutions as well. At the present time, we conclude that when growth rate is high and there is relatively little variation in bodyweight, rearing conditions are optimal. When growth is below expectation and variation is high coupled with poor feed conversion, social conditions within the pen are probably sub-optimal and pigs low in the social order perform poorly. (eg. Limited access to feed.) When growth rate is below expectation and variation is normal, then some aspect of the physical environment, such as crowding or poor diet, is depressing performance, and all pigs are affected uniformly.

Minimizing the Impact of Variability

Given our current state of knowledge, the most practical strategy would appear to be to manage the herd to minimize the impact of variability, rather than attempt to reduce variability per se. For example, depending on the grading grid employed by a given packer, time spent sorting pigs at marketing may result in a greater increase in net income than any other activity. In other words, hitting the range of carcass weights that maximize index, and avoid penalties for light and heavy carcasses, has increased average index on some farms by up to 4 points with a net value of $5 per pig sold.

Another strategy is to house barrows and gilts in separate pens (in continuous flow barns) or in separate rooms or buildings (in all-in-all-out facilities). In this manner, barrows will reach target market weights 7 to 10 days sooner than gilts; barrow pens/rooms can therefore be turned faster than gilt pens/rooms, with attendant increases in facility utilization.

Table 2 summarizes the bodyweights of over 400 pigs individually weighed at various intervals from birth to market and sorted according to the week in which they reached market weight. The data clearly shows the disparity in weights that can be traced back to weaning, and indeed to birth; the extent of this variation surprised us. The final strategy for minimizing the impact of variability, then, is to impose management practices that raise the growth curve for all pigs from the fastest growing to the slowest. In this way, variation may not be reduced, but with better overall performance, rooms or pens can be emptied according to schedule and the best market weights achieved in the grow out time available. All of the factors that we know so well will impact performance, such as overcrowding, inadequate diets, poor temperature control and poor health, should be minimized if not avoided in order to achieve an overall performance that maximized net income.

Take Home Message

Variation in bodyweight is an innate characteristic of all populations of animals. While variability is essential in breeding programs, it creates operational difficulty, especially during the grow out phase of production. It is now known that sorting does not improve uniformity of performance of a group of pigs, nor does reducing or increasing group size or increasing floor space allowance. Given the current state of our knowledge, the best strategy appears to be to minimize the impact of variation, rather than trying to change the amount of variation. Careful marketing, separate sex housing and increasing the overall performance of a group of pigs are likely to pay much bigger dividends than anything that can be done to reduce variation. Indeed, the benefits of such improvements will be measured in dollars, not cents, per pig!

Source: Prairie Swine Centre - June 2005