Comparisons of in situ nitrogen disappearance kinetics of wheat forages harvested by various techniques and evaluated in confined and grazing steers

Ruminal in situ disappearance kinetics of nitrogen (N) in forages are frequently evaluated in confined animals, but there have been relatively few attempts to obtain these kinetic estimates from grazing animals. Our objectives were to evaluate the effects of harvest technique and sampling date on the in situ N disappearance kinetics and nutritive value of wheat (Triticum aestivum L.) forage, and to compare in situ disappearance kinetics of N for these forages when the experimental steers were housed in confinement with those obtained from steers grazing wheat pasture. Wheat forages were harvested on three dates (March 6, March 27, and April 11, 2000) using five sampling techniques. These included three clipping techniques (whole plant, random pluck, and top half) and two evaluations of masticates (oven dried at 50 °C or lyophilized). Disappearance kinetics of N for the 15 experimental forages were evaluated by the in situ technique using five 393±54.4 kg crossbred steers that were offered an alfalfa based (Medicago sativa L.) diet and were housed in confinement. Subsequently, this evaluation was repeated in five 448±48.6 kg crossbred steers grazing wheat pasture during March 2001. Kinetic parameter estimates obtained from grazing steers were regressed on those obtained from confined steers. A slope of unity and an intercept of zero is indicative of agreement between the two methods. For lag time and fractions A and B (i.e., portions of total N that are immediately soluble and that disappear at a measurable rate, respectively), the slopes and intercepts did not differ (P≥0.126) from unity and zero, respectively. For the disappearance rate (Kd), the slope (0.92) did not differ (P=0.408) from unity, but the intercept (0.027) was higher (P=0.026) than zero. The slope (1.23) and intercept (−24.3% of N) for estimates of effective ruminal disappearance of N differed (P≤0.033) from unity and zero, respectively, but these differences were largely an artifact of the different ruminal passage rates used to calculate effective disappearance. From a practical standpoint, the in situ disappearance kinetics of N for these wheat forages were not altered substantially when they were obtained from grazing steers.