Variation in supplement intake by grazing beef cows

Abstract

One hundred twenty-one pregnant Angus cross cows (3 to 9 years of age; avg. wt 636 kg ± 50 kg) grazing native range pastures (Agropyron spicatum, Festuca idahoensis) were used to determine effect of herd size and cow age on individual supplement intake of a hand-fed pelleted protein supplement, variation in supplement intake, individual forage intake, and performance. The study was conducted at the Montana State University Red Bluff Research Ranch near Norris, MT from October 14, 2002 to December 13, 2002. Treatment was herd size, with seventy-six cows assigned to a large herd and forty-five cows assigned to a small herd. Each herd was assigned to one of two native range pastures to achieve equal stocking rates (0.4 AU/ha). Titanium dioxide was added to the supplement at 1% as an external marker to estimate individual supplement intake. Individual fecal samples were collected on d 23, 36, 38, 40, and 61 to obtain five measurements of individual supplement intake. Forage intake was estimated using estimates of fecal output obtained using chromium boluses and in situ 48 h DM digestibility. Forty frames per pasture (0.25 m2) were estimated and ten frames per pasture were hand-clipped to determine forage production and quality. Individual forage and supplement intakes were estimated and analyzed using individual animal as the experimental unit. Forage intake was higher (P = 0.05) for cows in the large herd vs. cows in the small herd (19.8 vs. 18.3 kg.cow-1.d-1). Herd size did not affect ADG (P = 0.11) between treatments (avg 1.06 kg/d gain); however, ADG was lowest (P < 0.001) for 3-year-old cows (0.72 kg/d) and highest for 7-year-olds (1.33 kg/d). There were no differences (P > 0.05) between herds for supplement intake on d 23, 38, 40, or 61; however, supplement intake on d 36 was higher (P = 0.03) for the large herd vs. the small herd (0.99 vs. 0.82 kg.cow-1.d-1, respectively). Average DM supplement intake was 33% lower (P < 0.001) for 3- and 4-yr-old cows compared to 8- and 9-yr-olds (0.72 vs. 1.07 kg/d, respectively). Supplement DMI CV was not different (P = 0.35) between herds (avg. 21%). Results of this study indicate that cow age may have more influence on individual supplement intake than herd size. Three-year-old cows had the lowest supplement DMI and the lowest ADG compared to all other age groups.