Development of an Efficiency Ranking System for Beef Cows and Effects on Feed Intake, Ruminal Fermentation and NDF Turnover, and Apparent Total Tract Digestibility

Abstract

Beef cows (n = 100) were ranked for efficiency based on cow rump fat thickness at calving, calving date, and calf weaning weight (% dam BW) over 2 years. The 9 most (ME) and least efficient (LE) cows were used to compare feed intake and ruminal fermentation using four 26-d periods with decreasing dietary nutrient density. There were no phenotype × diet interactions for variables of primary interest. Rump fat and calf weaning weight were greater and calving date was earlier for ME than LE (P ≤ 0.032). The ME cows were lighter (P < 0.001) but had similar DMI (P = 0.93) to LE cows, resulting in greater DMI (%BW; P < 0.001). Ruminal contraction amplitude height and area (P ≤ 0.015), and ruminal digesta weight were greater for LE than ME cows (P = 0.043). Ruminal aNDFom passage was greater for ME cows than LE cows (P = 0.047) but the rate of aNDFom degradation did not differ (P = 0.69). Total tract digestibility did not differ. Efficient cows had greater rump fat, weaned heavier calves, ate more relative to their BW, had smaller ruminal digesta mass, and greater ruminal passage of aNDFom without reducing digestibility.