Forage intake responses to winter cold exposure of free-ranging beef cows

Abstract

Winter foraging behaviors of pregnant crossbred beef cows grazing Montana native rangeland were examined over a continuous 46-day period in January and February. The objective was to examine daily estimates of two principle grazing behaviors, forage intake and time spent grazing, in response to fluctuations in acclimated thermal environments. Sixteen 5-year-old cows (525–575 kg) were fitted with vibracorders to monitor daily grazing time (DGT). Fecal organic matter output was estimated using total fecal collections and a Cr 2O 3 dilution technique. In vitro organic matter disappearance (IVOMD) was determined from extrusa obtained from four rumen-fistulated cows. Mean daily forage intake expressed as a percentage of body weight day (% BW per day) increased ( P<0.05) when present day ambient air or wind-chill temperatures deviated (either an increase or a decrease) from temperature averages of 1, 2 or 3 previous days, but the magnitude of response was small (<0.0005% BW per day per °C deviation). Daily intake was unresponsive ( P>0.05) to deviations from acclimated temperatures calculated for the previous 4–20 days. Dietary extrusa IVOMD averaged 34.5% and remained consistent ( P>0.05) during the winter grazing period. Daily grazing time decreased ( P<0.05) with ambient or wind-chill temperature deviations from acclimated thermal regimes of the past 1–3 days, and the magnitude of response was also small (<0.01 h per day per °C deviation). The slight responses of these two principal foraging behaviors indicated thermal fluctuations (8 to –16°C) within a familiar winter environment were minimally stressful with no resulting adverse effects upon the animal.