Comparison of Nitrogen Solubility in Three Solvents to In Vitro Protein Degradation of Heat-Treated Cottonseed Meal

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Abstract
Solubility of N in McDougall''s buffer 10% Burroughs'' buffer and 0.02 N NaOH was measured on samples of autoclaved and commercial cottonseed meal. Abilities of solvents to solubilize N were compared to ruminal protein degradation previously estimated by an in vitro procedure. Solubility of N in all 3 solvents decreased with increased heat-treatment. N extracted by 0.02 N NaOH was consistently greater than that extracted by the buffer solutions. With autoclaved meals, N solubilities in 0.02 N NaOH and McDougall''s buffer were 2.2-5.6 and 1.1-1.9 times greater than in Burroughs'' buffer. Linear regression equations were developed by regressing observed degradation on N solubilities for the autoclaved cottonseed meals. Degradation was more highly correlated with N solubility in 0.02 N NaOH than in McDougall''s and Burroughs'' buffers (coefficient of determination = 0.98 vs. 0.84 and 0.64, respectively). These regression equations were used to predict degradation of commercial cottonseed meals. Mean differences between predicted and observed degradation were 5.4, 6.7 and 12.9 percentage units for 0.02 N NaOH, McDougall''s buffer and Burroughs'' buffer, respectively. More realistic estimates of ruminal protein degradation were obtained from N solubilities in 0.02 N NaOH and McDougall''s buffer than in Burroughs'' buffer.