Effect of dietary protein level and quebracho tannin on consumption of pine needles (Pinus ponderosa) by beef cows

Abstract

<h2>ABSTRACT</h2> Ponderosa pine trees occupy over 15 million hectares of rangeland in western North America. Pregnant cows often consume pine needles (PN), and subsequently abort. The protein-to-energy ratio may be important in the ability of cattle to tolerate dietary terpenes. Tannins often co-occur with terpenes and may also influence diet selection. The objective of this experiment was to determine if the protein-to-energy ratio or the addition of quebracho tannin to cattle diets would influence PN consumption. In trial 1, 15 cows in moderate body condition were assigned to high (15.4% CP), medium (10.2% CP), or low (4.9% CP) dietary protein treatments for 12 d. In trial 2, 15 cows were assigned to high (5%), medium (2.5%), or no (0%) quebracho tannin diets for 8 d. In both trials, green PN were offered at 0930 h for 90 min. There was a treatment effect (P = 0.05) and a treatment × day interaction (P = 0.01) for PN consumption, as cattle on the high CP treatment consumed more PN than cattle on the medium and low CP diets for 4 and 6 d, respectively. Initially, treatments did not differ (P > 0.09), but from d 3 to 9 cattle on the high CP treatment ate more PN than the other treatments. There was a day × treatment interaction (P = 0.002) for PN consumption; cattle on the 2.5% tannin treatment consumed less PN than did animals on the other treatments. Cattle are apparently unable to tolerate high quantities of PN terpenes on a low-protein diet. Tannins may influence PN consumption, but the mechanism is unknown.