Year of Publication


Degree Name

Master of Science (MS)

Document Type





Animal Science

First Advisor

Dr. Merlin D. Lindemann


The objective of these studies was to determine the optimum standardized ileal digestible tryptophan to lysine (SID Trp:Lys) ratio for growing pigs fed both U.S.–type (corn–soybean meal) and non–U.S.–type (wheat–barley) diets.

When feeding U.S.–type diets, average daily gain (ADG) and plasma urea nitrogen (PUN) concentrations improved as SID Trp:Lys ratios increased, yielding optimum ratios of 15.32% (P<0.001) and 15.25% (P<0.001), respectively. Averaging these resulted in an optimum SID Trp:Lys ratio of 15.29%. In non–U.S.–type diets, feeding increasing SID Trp:Lys ratios improved ADG and PUN, displaying optima of 15.99% (P=0.048) and 15.29% (P=0.054), respectively. This produced an average optimum SID Trp:Lys ratio of 15.64%. The final study included the other dietary essential amino acids (EAA) at a higher level, and ADG and PUN improved as SID Trp:Lys ratios increased, yielding optima of 17.93% (P<0.001) and 16.17% (P=0.009). This averaged to an optimum SID Trp:Lys ratio of 17.05%.

Overall, diet composition did not affect the Trp:Lys requirement of growing pigs, however, feeding the other EAA at higher levels resulted in a higher estimated optimum SID Trp:Lys ratio. The optimum SID Trp:Lys ratio is estimated to lie between 15.29% and 17.05%.