Effect of Maturity and Drying Temperature on Nutritional Quality and Amino Acid Availability of Normal and Opaque-2 Corn for Rats and Swine

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Abstract
Four experiments with pigs and one with rats were conducted to determine the effect of stage of maturity and drying temperature on the amino acid (AA) availability and true nitrogen digestibility (TND) of normal corn (NC) and opaque-2 corn (OC). An evaluation of methods for assessing AA availabilities based on growth, plasma AA and fecal AA responses was also made. For the rat trial, corn of both varieties was harvested at six stages of maturity (from day 45 to 79 post-pollination) dried to 14% moisture at 40 C and fed as the only source of protein. NC and OC for pigs were harvested on days 52 and 80 post-pollination and dried at 40 and 60 C for a total of eight treatments arranged in a 2 × 2 × 2 factorial design. Weanling pigs (averaging 4.5 kg) were used to determine protein quality and tryptophan and lysine availabilities (by means of growth and plasma AA responses) of the differently treated corns. Larger pigs (20 kg) were used for determining the availability of 13 AA and the TND of the same corn treatments by means of the fecal analysis method. None of the differences in protein quality for rats or pigs, or in availability of AA and TND for pigs due to stage of maturity or drying temperature were significant for either NC or opaque-2 corn. However, a trend for better gains and feed conversion of pigs fed the immature corn compared to those fed mature corn was observed. Gains and feed conversions of rats and pigs fed OC were improved (P<.01) as compared to NC fed animals. Higher AA availabilities and TND were also observed in OC vs NC, even though threonine availability was the only value to show a difference (P<.05) due to corn variety. Lysine was the least available AA in both NC and opaque-2 corn. Tryptophan availabilities were relatively high in both NC and OC (92 and 95%, respectively). TND was also higher in OC (92%) than in NC (90%). A significant interaction (P<.05) of maturity X drying temperature was observed in the availability of eight amino acids. The interaction was attributed to the initial moisture level during drying: 38% for the immature corn and 18% for the mature corn. Drying high-moisture corn at 40 C resulted in higher AA availabilities and TND than drying at 60 C. The reverse was true for low-moisture corn which showed higher AA availabilities and TND from drying at 60 vs drying at 40 C. It appeared that high moisture corn is more susceptible to protein damage due to heat treatment than low-moisture corn. A 25% reduction in yield was noticed in both corn varieties harvested on day 52 post-pollination compared to the yield of mature corn. The fecal analysis method for calculating AA availabilities was more convenient and produced more reliable results than the growth assay method or a method based on plasma AA responses. Copyright © 1978. American Society of Animal Science . Copyright 1978 by American Society of Animal Science