Studies on methods for dry matter, crude ash, and nitrogen analysis in animal feeds
Authors:   , Quirino Daiana Francisca 
Published: May 2023

This study encompassed three experiments to evaluate different methods for quantifying the dry matter, crude ash, and nitrogen contents in animal feeds. In the first experiment, our objective was to evaluate the accuracy, repeatability, and physicochemical impacts of oven- drying times on laboratory dry matter estimates by loss on drying at 105°C. We evaluated eighteen different feeds, which were clustered into high- and low-moisture materials. The tested methods were based on loss on drying at 105°C and the Karl Fischer titration was adopted as the reference method. Test portions were oven dried at 105°C for 3, 6, 12, 16, and 24 h, and results were compared to the reference method. The method based on 3 h of drying provided the closest estimates to those obtained by Karl Fischer titration. The method based on the binomial 105°C × 3 h minimizes the bias in the water recovery, and it is recommended for dry matter analysis in animal feeds. In the second experiment, we evaluated methods for crude ash quantification. Our objective was to evaluate the influence of variations in time and temperature and the use of ashing aids as well, on the crude ash estimates. Twenty-four study materials from eight different feeds were evaluated. The control method was based on a simple ignition time of 3 h at 550°C. The methods variations were: increasing ashing time to 6 h; increasing temperature to 600°C; and using two 3-h ignition cycles at 550°C with ashing aids inclusion between them. The ashing aids were: fresh air supply, fresh air supply plus distillate water, and fresh air supply plus hydrogen peroxide. The crude ash results differed among the method variations, but a consistent decrease in the estimates was observed when liquid aids were applied. The best test results are obtained using water as liquid ashing aid between two ignition cycles at 550°C for 3 h. In the third experiment, we aimed to evaluate the nitrogen recovery in tropical forages by 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. The Kjeldahl standard procedure was based on digestion with sodium sulfate and copper sulfate (20:1), followed by steam distillation and titration. The Kjeldahl variations were: the Kjeldahl method with salicylic acid as pre-treatment; the Kjeldahl with titanium dioxide replacing 50% of copper sulfate in the digestion mixture; and the Kjeldahl with salicylic acid and copper sulfate:titanium dioxide (1:1) as a catalyst. We adopted the Dumas method as a standard method. Forty-eight study materials (i.e., tropical forages) were evaluated. Nitrogen recovered was not complete for the Kjeldahl methods, which did not differ to each other. Neither salicylic acid nor titanium dioxide improved the nitrogen recovery. The average nitrogen recovery in tropical forages when analyzed through the Kjeldahl method is 0.974 g/g. The nitrogen 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. Keywords: Animal feed. Animal nutrition. Feed analysis