Effects of ammonium nitrate and potassium sulfate fertilization on amino acid composition of tall fescue and orchardgrassi

In two experiments tall fescue (TF) and orchardgrass (OG) were fertilized in split applications with 0 or 882 kg NH₄NO₃/ha and 0 or 833 k₂SO₄/ha to determine the effects on TF and OG amino acid composition. Both free (FAA, unprecipitable in 6% trichloroacetic acid) and hydrolyzable (HAA, hydrolyzed in 6 N HC1 at 115 C for 22 hr) amino acid compositions were determined. Data were combined for regression analyses using data for herbage nitrogen (N) and sulfur (S) fractions from the same samples but previously reported (12). Ammonium nitrate fertilization changed herbage amino acid concentrations while K2SO4 fertilization had no effect. The relationship (P<.0001) between herbage total FAA and nonprotein N concentration in herbage was total FAA (pmol/g) = 17.24 + 2.045 (nonprotein N, mg/g) + 17.37, r₂ =.477. Furthermore, free cystine and methionine were linearly related (P<.001) to herbage nonprotein nitrogen content. Linear regression of total HAA (pmol/100 mg protein N) on total N content was significant (P<.002). The cubic equation improved the R2 to.338 and was total HAA (protein N basis) = -3778.7 (total N, mg/g) + 118.5 (total N)2 -1.2 (total N)3. Regression of total HAA content (pmol/g) on protein N content explained.490 of the variation and there was a significant (P<.0001) cubic effect (R2 =.580). Regression coefficients were negative for cystine and methionine concentrations (protein N basis) when regressed on total N content. There were no relationships between hydrolyzable S amino acids (pmol/g) and herbage N or S fractions. Ammonium nitrate fertilization increased (P<.0001) total FAA and total HAA concentrations which were nitrogenous components contributing to increased nonprotein N and protein N fractions, respectively.