Determining proper amino acid ratios in growing pigs' diets limits feed expense and reduces nitrogen excretion, according to a research report from the University of Kentucky.

If the required ratio is set too low, performance will be limited due to inadequate levels of amino acids in the diet. In contrast, if the required ratio is set too high, excess supplementation will occur, increasing cost and nitrogen excretion, says lead researcher Merlin Lindemann.

Lysine is the most limiting amino acid in most swine diets. Once that level is established, it is fairly easy to determine and fill other amino acid needs if the ratio to lysine is known. Researchers disagree about the proper ratio of tryptophan to lysine, however.

Another confounding factor is whether the ratio applies to total dietary amino acids, because not all dietary proteins are digested equally, nor are all amino acids absorbed equally. Therefore, a ratio at the muscle level, where the majority of protein synthesis occurs, may not be the same as the ratio for total dietary amino acids.

To find out the applicable ratio for amino acid absorption, University of Kentucky scientists studied the required standard ileal digestible trytophan:lysine (SID Trp:Lys) ratio for growing pigs fed mainly a corn-soybean meal diet.

A total of 120 crossbred pigs averaging 57 lb. were placed on five separate diets and housed five pigs/pen. The dietary lysine level was set at 0.75% total lysine, which equated to a standard ileal digestible (SID) lysine level of 0.66% based on the ingredients that were used. The dietary treatments were increasing SID Trp:Lys ratios (from 12.43 to 18.42%) fed for 21 days.

Determination of the required ratios was based on growth performance (average daily gain) and plasma urea nitrogen (PUN), which is reduced when the proper ratio is attained.

Analysis of the data confirmed that proper responses were achieved for an optimum Trp:Lys ratio of 15.70 (Figure 1) for average daily gain and a plasma urea nitrogen of 15.64 (Figure 2). This produced a mean ratio of 15.67, based on the true (or standardized) ileal trytophan and lysine values.

When converted to a total dietary Trp/Lys ratio for the feed ingredients tested, the ratio becomes 17.0.

In sum, this study provides very clear agreement between the response measures for the SID Trp:Lys ratio for growing pigs fed mainly corn-soybean diets.

Researchers: Merlin Lindemann, Anthony Quant and Gary Cromwell, all of the University of Kentucky. Contact Lindemann by phone (859) 257-7524, fax (859) 323-1027 or e-mail merlin.lindemann@uky.edu.