Our objective in the grow-finish unit is to maximize revenues by marketing the majority of our pigs at optimal market weight. This is best achieved by actually knowing the weight of our pigs as they approach this target weight. If the number of pigs that we have to weigh to get a reliable estimate of their weight and the range in weight within the barn could be reduced to a reasonable number, then it would save time, money and aggravation to both pigs and people. Several options are available in which only a sample of pigs are weighed and that still provide a reasonable estimate of pig weight.

Because the coefficient of variation in the weight of grow-finish pigs is about 10% to 12% and their weight distribution is approximately normal, it can be calculated that the minimum sample size to be weighed is 4% of the pigs. But the smaller the population size, the greater the variation within the group and the more pigs and pens from which they have to be selected. So for small populations, weigh a minimum of 50 pigs and for large populations weight 4% of the pigs or 50 pigs, whichever is the largest. For example if there are 200 pigs, to get 50 pigs you have to weigh 25% of the population but if there are 1000 pigs you are only weighing 4% of the pigs, or 40 pigs. For small populations, randomly select about one-third of the pens and then randomly select two or more pigs from each pen to reach your sample size of 50 pigs. In large units, it has been shown that barns that have 40-50 pens, that weighing one pig from each pen will suffice. It has also been shown that weighing the first volunteer pig that comes out of the pen gives as good result as selecting a particular pig. This volunteer pig system (gate run) certainly is much easier to work. The average weight, and the range in weight, of the 4% sample or the 50 pig sample will give an estimate of the average weight of the population within 1.5 kg and will also give an accurate estimate of the range in weights within the group. It is helpful to spray paint the weight of each pig you weigh on its back after it is weighed and before it is returned to its pen.

When to Weigh

The best predictor of weight gain is entry weight. The heavier the pigs at entry into the grow-finish barn, the better will be their growth rate and the earlier they will go to market. In unsorted pigs, there will be an average of about 30% in weight range of the pigs at entry. From previous knowledge of days to market, an estimate can be made as to when the sample weighings should be made. For example, if average entry weight to the grow-finish barn is 30 kg, the weight range in the group could be 20 to 40 kg. If the heaviest pigs put on 6 kg gain per week, they will be about 100 kg after 10 weeks in the barn. So for this unit, the sample weighing could be done after pigs are 10 weeks in the barn. From sample weighings of many pigs, it has been shown that the standard deviation for finishing pigs at 100 kg is about 9.5 kg. Because the weights are normally distributed, we can assume that 33% of the pigs in this barn weigh between 90.5 kg and 100 kg and 33% will weigh between 100 kg and 109.5 kg. Another 15% will weigh between 81 kg and 90.5 kg. There will probably be a few even lighter and heavier than these limits of 81 kg and 119 kg.

But the vast majority (96%) will weigh between 81 kg and 119 kg. Because you have 50 pigs in the barn with weights written on their back, it should be easier to select all 36% of the pigs that weigh between 109.5 kg and 119 kg or more. If the pigs in the barn are growing at 5 kg to 6 kg per week, the next 36% of the pigs should be ready for market in two weeks time after weighing.

The next major decision to be made is - when will you close out the rest of the barn? Close-out should occur when the cost of maintaining the population does not justify the retention of the pigs. This will depend on the price of feed, market hog price and whether or not there are pigs waiting to fill the available space. If there is an adequate supply of new pigs to put into the facilities, then the barn should be closed out when the daily margin over feed cost drops below the average margin over feed cost. In other words you are making less with the remainder of the pigs than you could make with the next batch.

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