Comparison of equations to predict the metabolizable energy content as applied to lucerne

Abstract

Metabolisable energy (ME) intake determines productivity in ruminant production systems and estimates of the ME content of feeds underpin nutritional production models and feeding standards across the world. An inaccurate estimation of ME content of a feed means nutritional models and decision support tools are erroneous, resulting in significant variance in expected liveweight gain or carrying capacity of a pasture. Currently in Australia there are a range of equations used to estimate ME of feeds. Utilising appropriate ME equations suitable for Australian forages, in this case, lucerne, is vital for predictive modelling for production and for any required ration or supplement formulation. The current study compared 24 ME equations in lucerne. Lucerne samples were taken at four different vertical strata grown in field trials of plants of different heights to generate samples of differing nutrient quality.

This research demonstrated that different ME estimation equations generate different ME values for the same lucerne sample. This is exemplified with ME values ranging from 10.25 to 16.58 MJ ME/kg DM for a sample in the top strata, and 7.7 to 13.75 MJ ME/kg DM for another sample in the bottom strata. The Minson (1984) equation, ME (MJ/kg DM) = 0.157 DOMD + 0.059 CP – 1.073, appeared the best equation to use for lucerne according to its lowest SD. This was congruent to the ME equation analysis for forage sorghum samples (Lwin et al. 2022).

This study did not determine which equations were biologically correct, however ME equations based on a combined regression using DOMD and CP parameter were most suitable for use in both forage sorghum and lucerne. This work needs to be validated across multiple forages in Australia to develop ME feeding standards for wider and improved applications for the extensive grazing industry.