by: Clarence Froese, M.Sc., Director of Nutrition, Genesus Inc.
Individual nutrient requirements are generally derived and expressed in terms of total daily amounts, as opposed to dietary concentrations. For example, finishing pigs require a total of 20 grams of digestible lysine per day to support a daily growth rate of 1 kg (2.2 lbs.). To ensure accurate formulations, the nutritionist should know the expected level of daily gain and feed intake and then set the dietary concentration of digestible lysine accordingly.
So a finishing pig expected to grow at the rate of 1 kg (2.2 lbs.) per day and consume 3 kg (6.6 lbs.) of feed per day would require a diet that contains 6.6 grams of digestible lysine per kg (3 grams/lb.) or 0.66%. If the expectation is for the same growth rate but feed intake is only 2.5 kg/day (5.5 lbs./day), then the dietary concentration of digestible lysine will need to be increased to 0.8% in order to still satisfy the total daily requirement.
Feed intake and growth rate are both affected by the energy level of the diet. As the dietary energy concentration increases, feed intake decreases. This change is sometimes accompanied by improved growth rates if dietary energy concentrations reach high enough levels. Recently completed trials by Genesus, for example, showed a decrease in daily feed intake of 6% and an increase in daily gain of 3% when Genesus finishing pigs (50% Duroc, 25% Yorkshire, and 25% Landrace) were fed containing 4% added fat versus diets containing no added fat. Because feed intake decreases as the energy level in the diet is elevated, daily intakes of all nutrients will also decrease. It is thus essential to consider the dietary energy concentration when setting levels for various nutrients, including lysine, in finishing diets.
To maintain a constant intake level of nutrients while accounting for varying levels of dietary energy concentrations, requirements for key components such as digestible lysine are expressed in terms of a nutrient to calorie ratio (g digestible lysine:Mcal or MJ of net energy). This mechanism automatically ensures that the final dietary lysine level will be adjusted to satisfy the pigs’ total daily nutrient needs even as diet energy (and resulting feed intake) levels change.
This is illustrated in Table 1, which compares digestible lysine levels for Genesus finishing pigs fed corn-based (higher energy) vs barley/wheat-based (lower energy) diets over the weight range of 100-130 kg (220-285 lbs.). Note that digestible lysine levels calculated for the corn-based diets are higher than those based on the wheat and barley-based diets. This occurs as a result of a constant ratio (2.55) being multiplied by two different diet energy levels. Consequently, the total daily lysine intakes between the two diets will be similar even though daily feed intakes will be different.
This example illustrates the importance of applying a lysine: energy ratio when determining final dietary lysine levels. Setting these ratios is one of the first steps the nutritionist performs when formulating diets and is a key factor in determining animal performance as well as cost-effectiveness of the resulting diets. For this reason, nutritionists emphasize lysine: calorie ratio as opposed to dietary concentration when discussing appropriate digestible lysine levels in swine diets.
Table 2 illustrates the appropriate digestible lysine: net energy ratios recommended for various weight ranges of Genesus pigs. These ratios have been derived from detailed trials undertaken by Genesus over the past three years. Their application will ensure that Genesus pigs are fed to maximize income over feeding cost.
TABLE 1: CALCULATED SID LYSINE LEVELS FOR GENESUS FINISHING PIGS1 FED TWO ENERGY LEVELS
TABLE 2. RECOMMENDED SID LYSINE: NET ENERGY RATIOS FOR GENESUS FINISHING PIGS1