Phenotypic Selection of Efficient Beef Cows and Differences in Digestive Physiology.

This study evaluated beef cows identified as having high or low efficiency when managed in extensive systems and whether they differ in terms of digestive physiology. Nine high (HE) and low efficiency (LE) cows were selected from a group of 86 Angus cows using 2 years of selection data. Yearly ranks were assessed based on three weighted criteria: cow rump fat at calving; calving interval; and calf weaning weight. The 18 cows were then ruminally cannulated and used to evaluate their response to high (HQ; 41.0±0.78% NDF), medium-high (MHQ; 56.9±1.17% NDF), moderate (MQ; 62.5±3.40% NDF), and low-quality forage (LQ; 64.3±1.40% NDF) diets using four consecutive 26-d periods where diet quality progressively decreased by period for all cows. There was no phenotype'diet interactions. Start and end BW were 48 and 45 kg less (P< 0.01) and rump fat was thicker for HE than LE cows (P=0.02). Although total DMI did not differ (13.0 kg/d; P=0.93), DMI (%BW) was greater for HE (2.12%) than LE (1.97%, P< 0.01). Ruminal pH, shortchain fatty acid concentration, and the molar proportion of acetate, butyrate, and propionate did not differ. Ruminal digesta weight was 4 kg greater (P=0.04) and the pool sizes of DM (P=0.01), NDF (P=0.03) and undigested NDF (P< 0.01) were greater for LE than HE. Ruminal NDF passage rate was greater for HE than LE cows (3.11 vs. 2.89%/h, P=0.05), but there were no differences for the rate of NDF digestion. Ruminal contraction amplitude (P=0.01) and peak area (P< 0.01) were higher for LE than HE cows. Apparent total tract nutrient digestibility did not differ (P≥0.23). Cows identified as being efficient under an extensive management system have greater DMI as a proportion of BW, less ruminal digesta weight, and greater NDF passage rate but did not differ for total tract digestibility. [ABSTRACT FROM AUTHOR]