By Minoy Cristobal, Su A Lee and Hans H. Stein, University of Illinois

Soybean meal is the major source of amino acids in most diets for pigs throughout the world and in the United States, nearly 20% of all SBM produced is included in pig diets. However, whereas a diet based on SBM and corn can easily be formulated to meet requirements for all digestible indispensable AA, feed-grade synthetic or crystalline AA can also be used to meet some of the AA needs, which results in reduced inclusion of SBM and increased inclusion of corn in the diet.

Because corn is believed to contain more net energy than SBM, increased usage of corn and synthetic AA and reduced usage of SBM is expected to result in increased NE in the diet. However, results of recent experiments indicate that the NE of corn is not greater than in SBM, which would indicate that diet NE will not be increased if SBM is replaced by corn and synthetic AA. However, this hypothesis has not been experimentally verified. Therefore, an experiment was conducted to test the null-hypothesis that diet NE does not increase if SBM inclusion is reduced and the inclusion of synthetic AA and corn is increased.

Diets

Six diets were formulated to meet all nutrient requirements for growing pigs including requirements for standardized ileal digestible indispensable AA. Two normal-protein diets based on corn, SBM, and crystalline lysine, or corn, SBM, and synthetic lysine, methionine, and threonine were formulated. These two diets contained 17 and 14% crude protein, respectively. Four additional diets containing 13, 12, 11 or 10% crude protein were formulated by gradually reducing the concentration of SBM in the diets and including increased concentrations of synthetic lysine, methionine, threonine, tryptophan, and valine in the diets.

Animals and experimental design

A total of 24 growing pigs (initial BW: 29.9 ± 2.4 kg) were housed in six indirect calorimeter chambers and allotted to the six diets using a replicated 6 × 6 Latin square design with six chambers, six periods and six diets. Thus, there were six replicate chambers per diet in the experiment. Pigs were housed in groups of four pigs per chamber. Each chamber was equipped with a stainless steel wet-dry feeder, and an auxiliary nipple waterer was available in each chamber to ensure free access to water. Each chamber was also equipped with a slatted floor, stainless steel fecal screens and urine pans, which allowed for total, but separate, collection of feces and urine. The temperature and relative humidity inside the chambers were controlled and maintained at the same level in all chambers.

Throughout the experiment, pigs were allowed ad libitum access to feed. Diets were fed for 13 days, where the initial 7 d were considered the adaptation period to the diet followed by 6 d of collection of feces and urine. Total heat production was calculated using analyzed oxygen consumption and carbon dioxide and methane production. Fasting heat production was also measured. Concentrations of digestible energy, metabolizable energy, and NE and apparent total tract digestibility of gross energy in diets were calculated.

Results indicated that feed intake, fecal and urine gross energy output, apparent total tract digestibility of dry matter and gross energy as well as total and fasting heat production and retained energy were not different between pigs fed the 17 and the 14% crude protein diets. Digestible and metabolizable energy in the 17% crude protein diet was greater (P < 0.05) than in the diet with 14% crude protein, but NE was not different between these 2 diets. Feed intake, gross energy intake and fecal gross energy output of pigs linearly (P = 0.044) increased by reducing crude protein in the diets, but reducing diet crude protein did not affect digestibility of dry matter and gross energy or urine gross energy excretion. Total and fasting heat production, retained energy, digestible energy, metabolizable energy or NE were also not affected by reducing crude protein in the diets. The calculated NE in the diets ranged from 2,605 to 2,665 kcal/kg and did not increase or decrease as diet crude protein level was reduced. These observations indicate that the NE in SBM likely is close to that in corn, which is in agreement with some recent observations from commercial swine production units.

In conclusion, reducing crude protein of diets for pigs by reducing SBM and increasing the concentration of synthetic AA and corn did not increase NE of diets when fed to group-housed growing pigs that were allowed ad libitum access to feed, and dietary CP does not appear to influence diet NE. It is also concluded that the NE concentration in SBM is greater than previously reported. Funding for the current research from the United Soybean Board, St. Louis, Missouri, is acknowledged.