Variations of the Kjeldahl method for assessing nitrogen concentration in tropical forages

Nitrogen (N) is one of the most important components of feeds and its quantification allows estimating protein equivalents, an important characteristic for diet formulation in ruminant nutrition. We aimed to evaluate N recovery in tropical forages using the Kjeldahl method with modifications involving a pre-digestion step with salicylic acid and, or a partial replacement of copper sulfate by titanium dioxide as a catalyst. Forty-eight study materials (i.e., tropical forages) were evaluated. Kjeldahl standard procedure was based on acid digestion with sodium sulfate and copper sulfate (20:1), followed by steam distillation in sodium hydroxide and titration with hydrochloric acid. The Kjeldahl variations were: salicylic acid as pre-treatment before digestion, titanium dioxide replacing 50% of copper sulfate as catalyst in the digestion mixture, and salicylic acid and titanium as described above. The Dumas method was used as standard method to verify N recovery through Kjeldahl procedures. The N concentrations of the study materials (Dumas method) ranged from 1.9 to 28.3 g/kg as-is. We found that all the methods were strongly and positively associated with each other (p < .01). All Kjeldahl methods produced N concentrations different from those obtained by the Dumas method (p < .01). The N recovery ranged from 0.971 to 0.980 for the different Kjeldahl methods. However, we did not find any difference between the different Kjeldahl methods regarding N concentrations (p≥ .89). The N recovery of the Kjeldahl method is not influenced by a pre-digestion with salicylic acid or by the partial replacement of copper sulfate with titanium dioxide as a catalyst.