In vivo digestibility of treated and untreated barley straw: results of direct and by-difference digestibility trials

A herd of 28 castrated adult male goats of the Murciano-Granadina breed was used to determine the direct and by-difference digestibility of untreated straw, straw treated with 4% urea and straw treated with 4% urea and 1% NaOH. four direct digestibility trials the diets consisted exclusively of alfalfa hay, untreated (with N supplementation), or treated straws. In the three by-difference digestibility trials we used alfalfa hay as a basic diet at a ratio of 50,50 and untreated or treated straw. Dry matter digestibility increased when the straw was treated (P < 0.001), but there were no differences (P > 0.05) between the methods for determining digestibility. An effect of methodology on crude protein digestibility (CPD) was found (P < 0.001) and the type of straw X methods interaction was also significant (P < 0.001). CPD was significantly greater in straw treated with urea (P < 0.001) than in straw treated with urea + NaOH or untreated straw. The digestibility of the straw fibrous fractions improved with the treatments. On calculating the by-difference digestibility of crude fibre (CF), it was noted that straw treated with urea + NaOH had higher (P < 0.01) digestibility coefficients than in the direct digestibility trial. The digestible organic matter (DOM) content of the straw improved (P < 0.001) with the treatments and was not affected (P > 0.05) by the method used to calculate the digestibility. However, in urea + NaOH-treated straw, the DOM content was lower (P < 0.05) when determined directly than when calculated by difference, due to associative effect. In general, a greater standard error was found when the digestibility was calculated by difference, but both techniques showed the effect of treatment on straw. However, in urea + NaOH-treated straw it is advisable that digestibility be calculated by difference, in order to avoid high concentrations of alkali from urea + NaOH-treated straw in the rumen.