Abstract | Tarbush (Flourensiacernua) represents a substantial biomass in the Chihuahuan Desert. Two studies were conducted to assess intake, in situ digestion, passage rate, ruminal fermentation, and nitrogen balance of sheep fed a low quality tobosa (Hilariamutica) diet containing up to 30% tarbush leaves. Tarbush was harvested and air dried, and leaves were separated from stems. In Exp. 1, 16 ruminally cannulated Polypay x Rambouillet lambs (avg BW = 46 kg) were individually fed (twice daily) ground (2.54 cm) tobosa (5.9% CP, 80% NDF; DM basis) substitued with 0, 10, 20, or 30% (DM basis) tarbush (18.9% CP, 33% NDF) leaves ad libitum for 28d. In Exp. 2, 24 Polypay x Rambouillet wethers (avg BW = 47 kg) in metabolism crates were allotted to one of the four treatments described above (n = 5) or 26% ground alfalfa (n = 4). Sheep were fed ad libitum for 11 d and then restricted to 1% of BW for 11 d to equalize intake. In Exp. 1, DMI for 20 and 30% tarbush treatments was greater (P = .0049) during week 3 than 0 and 10% treatments. Molar butyrate proportions (P = .0032) and total VFA concentrations (P = .0064) were greater for the 30% tarbush treatment than other treatments. Sheep fed 30% tarbush had greater (P < .0046) ruminal ammonia concentrations than sheep fed other diets at 6, 8, and 12 h postfeeding. In Exp. 2, all treatments resulted in similar but negative N retentions (-2.4, -2.2, -2.8, -2.0, and -1.5 g/d for sheep consuming 0, 10, 20, or 30% tarbush or alfalfa, respectively). Because only 30% tarbush elicited measurable responses in Exp. 1 (based on ruminal ammonia dn total VFA concentrations), nutrients contained by tarbush (especially CP) did not appear to be completely available to sheep. Low DMI observed in Exp. 2 was likely due to secondary compounds in tarbush and may partly explain why no differences in N balance were observed in Exp. 2. |