Forage intake: I. Forage intake of lactating beef cows differing in production; II. Methods of administering ytterbium for estimating fecal output

Two studies were conducted: (1) develop forage intake models for lactating cows with different milk production; and (2) investigate methods of administering Ytterbium (Yb) as a marker to estimate fecal output of confined and grazing animals. In the first study, 24 beef cows, eight from each production level (low, L; medium, M; and high, H) were grouped in pairs based on similar past performance and used in two 42-d feeding trials (early and late lactation) to measure ad libitum intake of chopped Sandhills meadow hay. Daily dry matter intake and milk consumption had a positive linear relationship (P $\u3c$.05) among L, M and H cows during both trials. Intake expressed as g/unit body weight had a positive linear (P $\u3c$.05) and quadratic (P $\u3c$.01) relationship. Three experiments conducted in the second study used grazing and penned animals to measure accuracy and precision of six methods of administering Yb for estimating fecal output. All animals were fitted with fecal collection bags and a paired difference test was conducted to determine differences between estimated and measured fecal output. Methods within an experiment were compared using estimated deviation from measured output (absolute value) in a one way analysis of variance. Confined sheep in Experiment 1, were orally dosed with either a pulse dose (P) or a once daily dose (C). Both method over estimated fecal output, however, the standard error for C was.78 compared to 2.17 for P. In Experiment 2, a cross-over design was used to compare output estimates by pulse dose (PR) and once daily dosing (CP) via rumen cannulas in steers grazing dormant forage. Although both methods were relatively accurate CR had a lower standard error (2.36) than PR (3.64). In Experiment 3, intraruminal constant release bolus (CRB) was compared to daily feeding of labeled supplement (YLS) using grazing and pen fed steers. The CRB overestimated fecal output 14.8% on pasture and 41.3% in the pen study. Standard error was as much as 5.67. Although YLS did not give an accurate estimate of fecal output (80. 1 and 106.0% of measured on pasture and pen, respectively), it had a lower standard error than CRB during both trials. (Abstract shortened with permission of author.