Factors Influencing Pine Needle Consumption by Grazing Cattle during Winter

Abstract

Ponderosa pine (Pinus ponderosa Lawson) needles cause abortions in pregnant cows. We examined pine needle consumption by cattle in 2 trials in eastern Montana. Trial 1 compared pregnant and open cows (n=4) from January to March 1989; trial 2 compared pregnant cattle (n=4) that received either 9 kg alfalfa hay head-1 day-1 or 1.4 kg alfalfa pellets head-1 day-1 from December 1989 to February 1990. Diets were estimated using both bite counts and fecal analysis. During trial 1, bite counts revealed pregnant and open cows consumed 45 and 42% of their grazing diets as pine needles (P>0.1). Fecal analysis showed that pregnant cows consumed more pine needles than did open cows (36% vs. 27%, respectively) (P<0.05). During trial 2, cattle consumed < 1% of their diets as pine needles. In trial 1 cattle consumed less pine litter and consumed more needles from trees as snow depth increased. Consumption of needles from trees increased as ambient temperature declined; no needles were consumed from trees when the minimum daily temperature exceeded -5 degrees C. During both trials, grazing times decreased as temperatures declined, and increased as snow depth and wind speed decreased. We conclude that weather is a major factor influencing needle consumption; other interrelated factors may be forage availability, snow cover, and grazing time. Pine needle consumption, and the risk of abortion, in pregnant cattle appears to be greatly diminished during mild winter weather.