Digesta markers for evaluating the effect of exogenous enzymes on nutrient digestibility in beef feedlot cattle

Abstract

The Food and Agriculture Organization (FAO) stated that it is vital to intensify animal and feed production in a sustainable manner. Producers are therefore increasing focus on methods to improve production efficiency. The use of exogenous enzymes in ruminant diets can improve production efficiency while also reducing waste products, thereby contributing to more sustainable food production. There are many combinations of enzymes that first have to be screened through in vitro methods, thereby selecting a number of enzyme combinations with the highest potential effectiveness. Ultimately, however, the best combinations need to be validated using in vivo digestibility studies. Ruminants, in general, have variable responses to supplementation with exogenous enzymes and the animal response to different enzymes can be influenced by the type of enzyme or combinations used as well as the experimental conditions. The in vivo validation of prototype enzymes by means of digestibility studies is therefore of utmost importance before commercialisation of a product. To measure total tract digestibility the total faecal collection method is normally used. Total faecal collection is, however, labour intensive because all the faeces produced in a period of time must be collected and the animals must be confined to individual housing which may disturb the animals. Marker-based methods are less labour intensive are an attractive alternative to the total faecal collection technique. Before any marker can be used in digestibility studies, it should be validated to confirm the suitability of the specific marker in a specific diet. The aim of this study was first to evaluate the effect of different enzyme prototypes on feedlot diet digestibility and secondly to validate different markers as an alternative to total faecal collection in digestibility studies using high maize feedlot diets. Six ruminally cannulated steers were used to evaluate the apparent total tract nutrient digestibility using two different enzyme prototypes against a control diet. The steers were fed twice daily and received either the control diet or a control diet supplemented with either enzyme A or enzyme B. Three different markers, chromic (III) oxide (Cr2O3), acid insoluble ash (AIA) and acid detergent lignin (ADL) were validated against total faecal collection to determine which marker is most suitable in digestibility studies for feedlot diets containing high levels of maize (60%) and low levels of roughage (20%). The mean apparent total tract digestibility (TTD) for dry matter (DM), starch and crude protein (CP) showed no difference (P>0.05) between the control diet and diets supplemented with enzyme A or enzyme B. The neutral detergent fibre (NDF) digestibility did show a difference (P<0.05) between the control diet and the diet supplemented with enzyme B. However there were no differences (P>0.05) between the control diet and the diet supplemented with enzyme A or between the two diets supplemented with the enzymes A and B. Acid insoluble ash and Cr2O3 showed no difference (P>0.05) between treatments in its ability to predict digestibility when compared to total faecal collection. Acid detergent lignin predictions differed (P<0.05) from total collection and is probably unsuitable to be used as a marker in high maize feedlot diets. The use of enzymes containing xylanase, amylase and protease had no effect on nutrient total tract digestibility in our study; however, the enzyme-containing xylanase and β-glucanase had an effect on apparent total tract NDF digestibility. Both AIA and Cr2O3 can be used as markers to determine apparent total tract nutrient digestibilities in feedlot diets with a high maize content based on a comparison with the total faecal collection. Acid detergent lignin appears to be a poor marker in high concentrate feedlot diets due to the low ADL content of the diet.