Introduction
The particle size of SBM from the processing plant averages approximately 900-microns in size. Dietary particle size has been demonstrated to affect the digestibility and utilization of certain nutrients, particularly amino acids, and a size of 600 to 700-microns is recommended (Owsley et al. 1981). When an ingredient is ground the particles become smaller thus increasing the surface area allowing the digestive enzymes to be more effective. Consequently, digestibility is increased as particle size is reduced. The processing of SBM to a smaller particle size may also increase the digestibility of its amino acids.
Although SBM has generally been considered to be an excellent quality protein source, approximately 10 to 15% is not digested. This undigested fraction is excreted and contributes to the nitrogen pollution of water reservoirs. Therefore, any method of increasing the digestibility of SBM would not only improve swine performance but may also reduce the amount of nitrogen pollution.
Amino acid digestibility measurements have traditionally used fecal and urine collections. Microbes in the large intestine, however, metabolize amino acids thus making fecal digestibility a relatively inaccurate measurement. More recent research has evaluated the effects of determining the digestibility of feed ingredients from the end of the small intestine (ileum). The digestion and absorption of amino acids are completed before this point. Therefore, ileal digestibility can eliminate the effects of the confounding problem of the microbial population in the large intestine. Measurements at the end of the small intestine should therefore more accurately reflect ingredient digestibilities.
There are amino acids present in the digesta from the ileum that are not of feed origin. They constitute the endogenous fraction and originate from the sloughed cells of the lining of the intestines, digestive enzymes, and other body sources. The pig continually excretes these endogenous secretions thus contributing to the intestinal digesta.
In order to adjust for these endogenous secretions it has become a common practice to experimentally feed a low-protein diet for several days and collected the digestive secretions. We formulated a diet with 5% casein and thus calculated the digestibility by removing the endogenous portion of amino acids at the end of the ileum and subtracted them from the amount present in the diet. We assumed the casein-fortified diet to be 100% digestible and therefore the amino acids present in the digesta are considered endogenous.
This experiment evaluated the effect of various SBM particle sizes on low-protein adjusted ileal digestibility of amino acids in the grower-finisher pig.