By Mark Knauer and Jeff Wiegert, North Carolina State University
Pig farmers recognize the importance of pig weight. The economics of marketing decisions, dietary phase changes, deciding when to mate gilts, etc., are all closely tied to an approximation of pig weight. Yet available tools to capture pig weight, without the use of a scale, are limited. To our knowledge we are still waiting for a cost-effective, handheld device that accurately estimates pig weight. Yet we are confident this technology will be developed in the future.
In relation to simple body measurements, heart girth circumference (Figure 1) is perhaps the most accurate way to estimate pig weight. Yet anyone who has tried to measure the heart girth of a pig knows that using a weigh tape can be challenging. Hence we have developed an improved method of measuring heart girth circumference by using a 3/8-inch polyethylene tube (Figure 2) as an alternative to using a weigh tape. The tubing provides the durability and flexibility needed to increase ease of measurement.
Figure 1: Desired location to measure the heart girth of pigs. (Diagram from ThePigSite)
Figure 2: An example of an individual measuring pig heart girth with a tube.
We conducted an observational experiment to validate the accuracy of the heart girth tube. Data was collected from pigs (n=101) in a commercial farm at an average weight of 256 ±32 pounds. Both pig weight and heart girth circumference were measured once. The correlation between pig weight and heart girth was 0.93 indicating that heart girth circumference is a very good predictor of pig body weight. Heart girth accurately predicted 50% of the pigs within nine pounds and 80% of the pigs within 16 pounds of their live weight. Yet we hypothesize the correlation between heart girth with carcass weight may be greater than that with live weight due to differences in pig gut fill. The regression equation developed for estimating live weight was: live weight = -269.365 + heart girth circumference in inches × 11.546. This equation is similar to the one developed by the Kansas State University Swine Extension Team (Groesbeck, et al., 2002, Figure 3).
Figure 3: Regression equations estimating pig weight based on heart girth circumference from North Carolina State University and Kansas State University (Groesbeck, et al., 2002).
Where can the heart girth tube be best utilized within pig farms? We believe there are several areas it may be beneficial.
1). Niche farms or youth projects where it is perhaps not cost-effective to own a weigh scale. We often hear of niche producers having challenges marketing pigs at the correct weight.
2). Ensuring proper gilt weight at first mating in sow units.
3). Identifying pigs for market in farms that have excessive sort loss. We do not suggest measuring every pig in the finisher. Yet there may be value to using the heart girth tube on a few pigs per pen, writing the estimated weight on the pigs, and using them to calibrate one’s eye. It is perhaps easier to use the heart girth tube standing on the outside of the pen as pigs are generally accustomed to people in the alleyways more so than in the pen.
4). Evaluating a farm’s stockmanship. We have been in gilt development units and commercial finishers where we were able to capture heart girth measures pretty easily and other farms where it was quite challenging. Given the genetics and nutrition among farms were identical, docility differences between farms indicate how often people saw the pigs and/or how people interacted with the pigs.