The industry is increasingly using information on lean tissue gain to determine nutrient requirements of grower-finisher pigs. Therefore, a working knowledge of how to determine lean growth rates for a particular herd is essential when using any type of modeling or factorial approach, including NRC (1998), to determine nutrient requirements for growing-finishing pigs. There are many different ways to determine lean tissue gain, including the use of packing plant data obtained from the final carcass to the use of realtime ultrasound. This article will give a review of a method using packing plant carcass data that can be used on any farm.

The determined lean tissue gain over the grower-finisher period can then be used in any model, including NRC (1998), to determine the nutrient requirements for that particular herd. To determine lean tissue gain over the grow/finish period, the initial lean subtracted from the final lean gives an estimate of lean tissue gain during the grower-finisher period. To get the average daily lean growth rate, divide the lean tissue gain by the number of days in the grower-finisher period. This value is then entered into the model to determine the nutrient requirements for that herd (or pig). More specifically, assuming that the lean content in a 25 kg pig is constant at 35% of live body weight gives the initial lean for the pig. Information from the packing plant provides the information needed to determine lean in the pig at slaughter. Taking the dressed weight and multiplying it by the % yield (obtained from the grading certificate from the packing plant) will give the lean in the carcass. However, 8 kg must be subtracted from the carcass lean to account for the weight of the head, feet, tail, kidneys and leaf fat. An example calculation using Prairie Swine Centre Inc. data is provided in Table 1.

Specifically for the NRC (1998) model, the lean growth rate together with the diet DE content of feed intake information are entered to determine nutrient requirements. Therefore, it is important to measure the feed intake of the same pigs that provide the lean growth data. Be aware of some factors to consider when using this method to determine nutrient requirements. First, this approach cannot estimate a nutrient requirement higher than the level fed to the pigs whose performance was measured. For example, the approach estimates the amount of lysine needed to reach the measured level of performance. If the requirement estimate is near the amount fed, it is possible that lysine intake is limiting performance, and that the pig is capable of growing more rapidly with more lysine. In that case, it is important to repeat the measurements with a higher dietary lysine level. Requirement estimates made previously may no longer be appropriate after significant changes in genetics, health, stocking density, temperature or other factors. Close monitoring of performance and nutrients supplied in the diet will maximize productivity of that herd.