Effect of different nitrogen sources on dry matter intake and digestibility of a low-quality roughage fed to sheep

Abstract

Extensive sheep farming in South Africa generally imply circumstances where roughage is the primary food source. However, when roughages form the main component in ruminant diets these diets are typically characterised by high fibre, low energy and low nitrogen values. Adding nitrogenous compounds to low-quality roughage not only promote the growth of fibrolytic bacteria, but also enhance digestion rates and microbial protein synthesis. Urea is the nitrogen source most often utilised for supplementing nitrogen as it is relatively inexpensive and proven to be effective. However, the rapid hydrolysis of urea to ammonia poses a large risk in that it can cause ammonia toxicity due to the accumulation of ammonia. Therefore it was the aim of this study to determine if using urea in smaller quantities but in combination with other N-supplements, would prove effective with regards to the following parameters: dry matter intake, rumen ammonia nitrogen, blood urea nitrogen, rumen pH, volatile fatty acid concentration, as well as dry matter and neutral detergent fibre digestibility. Four Merino wethers fitted with rumen cannulae were used in a 4 x 4 Latin square design and given a basal diet of ad libitum low-quality Eragrostis curvula hay. Supplements consisted of a mineral mixture, a nitrogen source (depending on treatment), as well as starch to ensure that the diets were iso-nitrogenous and iso-energetic. It was imperative to maintain a constant nitrogen:sulphur ratio throughout all treatments and therefore feed grade sulphur was included in the formulation of the treatments (where applicable). The required nitrogen was supplied as follows: Treatment 1 as control (U) - 100% urea; Treatment 2 - 50% urea and 50% (NH4)2SO4 (UAS); Treatment 3 – 50% urea and 50% lucerne (UL); and Treatment 4 – 8% urea, 84% (NH4)2SO4 and 8% lucerne (UASL). The results showed that animals fed UASL had a rumen ammonia concentration which was significantly (P < 0.05) higher compared to those fed the U treatment. In addition, the U treatment also proved to have the lowest DM and organic matter digestibility which differed significantly from the UASL and UAS treatments. At every measurement, the pH of the rumen fluid for the U treatment was higher compared to the UASL treatment, although these differences were significant at only a few intervals. No significant differences were found between treatments for dry matter intake, blood urea nitrogen, NDF digestibility, mean nitrogen retention, VFA concentration or the effective degradability of DM and NDF as determined by the in situ technique. These results did not reveal a clear effective partial replacer for urea as no single treatment performed better across all measured parameters compared to the control treatment (urea only).