A comparison of two enzymatic in vitro methods to predict in vivo organic matter digestibility of perennial ryegrass

Accurate grass organic matter digestibility (OMD) values are required for intake prediction, for calculating metabolisable protein and ultimately to accurately formulate diets. The three objectives of this study were: (i) to compare the neutral detergent cellulase (NDC)- and pepsin cellulase (PC)-predicted OMD to in vivo OMD; (ii) to compare the NDC method to the PC method for accurately estimating grazed grass OMD and (iii) to compare the effect of sample preparation on the NDC method results, specifically oven drying samples at 60 °C compared to freeze drying at −55 °C. Data consisted of 34 forage grass samples on which OMD was measured in vivo using the total faecal collection method in sheep (OMDINVIVO). The grass samples were also analysed in vitro in triplicate. For the NDC method the samples were oven-dried at 60 °C for 48 h or were freeze-dried at −55 °C for 1 week. The PC method samples were oven-dried at 60 °C for 72 h. The NDC organic matter solubility (OMS) results were inserted into the equation developed by Stakelum et al., (1988. Irish J. Agric. Res. 27, 104-105) to give OMDNDC. The OMS results from the PC method were inserted into the equation developed by Aufrère et al., (2007. INRA Prod. Anim. 20, 129-136) to give OMDPC. The comparison between OMDPC 60 (mean±standard error) (0.708±0.0361; n=34) and OMDINVIVO (0.719±0.0361) showed that there was a tendency for the two methods to give different results (P=0.06). The comparison between OMDNDC FD (0.740±0.0049; n=18) and OMDINVIVO (0.764±0.0063) showed that the two methods gave different results (P<0.01). The comparison between OMDNDC 60 (0.680±0.0265; n=34) and OMDINVIVO (0.719±0.0361) showed that the two methods gave different results (P<0.05). The positive correlation with OMDINVIVO suggested that OMDNDC FD, OMDNDC 60 and OMDPC 60 were able to predict OMDINVIVO of perennial ryegrass grazed swards. The comparison between OMDNDC FD (0.740±0.0049) and OMDPC 60 (0.746±0.0109) showed that the two methods gave similar results (P>0.05). The comparison between OMDNDC 60 (0.680±0.0230) and OMDPC 60 (0.708±0.0230) showed that the two methods tended to give different results (P=0.06). To improve the relationship between OMDINVIVO and OMDNDC FD, and between OMDINVIVO and OMDNDC 60, future work should expand the number of grass samples, the months of the year over which samples are collected and the range of chemical composition and OMDINVIVO of the samples. Sample preparation method had an effect on the NDC-predicted OMD results as OMSNDC FD differed to OMSNDC 60 and OMDNDC FD differed to OMDNDC 60. This indicated that specific correction (regression) equations must be used for specific sample preparation methods.