Substitution method for evaluating effective energy value of rice straw and wheat straw for beef cattle

This study aimed to estimate the effective energy value of rice straw and wheat straw for beef cattle using the substitution method, and to investigate the approximate substitution ratio. Twelve Wandong bulls with similar body weight (266±13.41) kg were used and randomly divided into three groups of four bulls each. We conducted three experiments. In Experiment Ⅰ, all animals were fed a whole-crop corn silage based diet and the effective energy value of the basal diet was determined. In Experiment Ⅱ and Experiment Ⅲ, animals were fed experimental diets consisting of the basal diet substituted with different ratios (10%, 30%, and 60%) of rice straw or wheat straw, respectively (dry matter (DM) basis). Each experiment lasted for 14 days, including a pretest period of 10 days and a test period of 4 days. A digestibility trial, integrated with a respirometry trial, was conducted to determine energy metabolism of all experimental diets for beef cattle. Compared with the basal diet group, rice straw or wheat straw substitution decreased the DM, crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF) digestibility of the experimental diets, decreasing gradually as the rice straw or wheat straw substitution ratio increased. There was no significant difference in gross energy (GE) intake among all the groups (P>0.05), whereas, the digestible energy (DE), metabolizable energy (ME), and net energy (NE) intake decreased as the rice straw or wheat straw substitution ratio increased (P<0.05). The GE digestibility, GE metabolized, and DE metabolized were also negatively correlated with the substitution ratio of rice straw or wheat straw. According to the regression equation between the effective energy value of different experimental diets and rice straw or wheat straw substitution ratio, the DE, ME, and NE values for rice straw were 8.51, 5.79, and 3.64 MJ·kg-1, respectively, and the corresponding values for wheat straw were 7.89, 5.08, and 3.13 MJ·kg-1. Compared with the other two substitutions, 30% rice straw or wheat straw had the least variability (P<0.05) in effective energy value. In conclusion, in determination of the effective energy value of rice straw and wheat straw using the substitution method, a significant difference was observed when the substitution proportion was different. In the current study, a rice straw or wheat straw substitution ratio of 30% was better than 10% or 60%. The combination of regression method and substitution method can improve the accuracy of the results when estimating the effective energy value of single roughage for beef cattle, and contribute to acquiring the optimal substitution ratio.