Retention time of digesta in the rumen: its repeatability and relationship with wool production of Merino rams.

The purpose of this experiment was to determine whether differences in retention time (RT) of digesta in the rumen are associated with individual differences in wool growth rate between Merino rams. Fifty-eight rams were offered a pelleted diet at ad libitum and restricted levels. Digesta RT in the rumen was estimated from fecal excretion plots using 51Cr-mordanted feed as a marker. Wool growth rate during feeding for ad libitum intake was linearly correlated to DMI (r = .37; P < .01) and RT (r = -.32; P < .01). Multiple logistic regression analysis, used to overcome multicollinearity, showed that RT per se exerted a greater effect (P < .07) than DMI (P < .34) on wool growth rate at ad libitum feed intake. When data were blocked by wool growth rate keeping mean BW constant over all groups, high wool producers had shorter (P < .01) RT than low wool producers. Using the probability values of the multiple logistic regression analysis (CATMOD), the probability of obtaining a low wool growth rate with long RT was estimated to be .73, whereas that of obtaining a low wool growth rate with short RT approximated zero, in the ad libitum period. During the restricted feeding period, the probability of obtaining a low wool growth rate with long RT was estimated to be .46, whereas that of obtaining a low wool growth rate with short RT was estimated to be .17. Repeatability, the upper limit of heritability, was .45 (P < .01) and .6 (P < .01), using regression correlation and Spearman rank correlation coefficients, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)