Use of an in vitro gas production procedure to evaluate rumen slow-release urea products

The bulk of the gas produced in the in vitro rumen gas production technique originates from bicarbonate buffer, which reacts with volatile fatty acids (VFA) to release CO2. Ammonia, which is produced from fermentation of nitrogenous substrates, neutralizes the VFA and prevents them from reacting with the buffer. We hypothesized that the in vitro gas test could be a suitable method to differentiate slow-release urea (SRU) products basing on the intensity of ammonia release and consequent reduction in gas produced. The study was composed by two in vitro gas experiments (i.e., Expt.'s 1 and 2), which were conducted using graduated 100 ml syringes with gas measurements at 1, 2, 4, 6, 8, 10 and 24 h of incubation (in Expt. 2, also at 0.5 and 18 h). In Expt. 1, 500 mg of corn (Zea mays subsp. mays) meal (CM), alone or added with four progressively higher urea doses (i.e., 25, 50, 75, 100 mg) were examined to evaluate the patterns of gas reduction and modifications of the fermentation process with urea addition. Increasing urea doses caused a consistent depression in gas produced at each measurement point (P < 0.01) and an increase (P < 0.01) in ammonia N concentration in the fermentation fluid after 24 h of incubation. When urea added exceeded 50 mg, a decrease (P < 0.01) of dry matter substrate apparent degradability (from 0.76–0.77 to 0.68-0.71) occurred. In Expt. 2 a group of 9 commercial SRU products, containing from 361 to 420 g of N/kg of dry matter, were added to 500 mg of CM in amounts equivalent to 50 mg (on an N basis) of feed grade urea (i.e., CM + SRU). Corn meal alone, CM added with urea (i.e., CM + U) and CM + SRU mixtures were incubated as in Expt. 1. At all measurement points, gas production was highest for CM and lowest for CM + U substrates (P < 0.01) while all CM + SRU mixtures were intermediate (e.g. from −40/50 to −70/80% of the CM gas yield, at 4 h). The in vitro gas procedure can be used to evaluate SRU products in terms of reduction of gas volumes due to the ammonia release. The procedure seems to be of high sensitivity because can well rank different commercial SRU products, mainly in the first hours of incubation. Further research might consider using the procedure by modern equipment and performing experiments to test accuracy and precision. © 2017 Elsevier B.V.