EFFECTS OF YUCCA-SHIDIGERA EXTRACT AND SOLUBLE-PROTEIN ON PERFORMANCEOF COWS AND CONCENTRATIONS OF UREA NITROGEN IN PLASMA AND MILK

Twelve multiparous Holstein cows averaging 122 d postpartum were used in a replicated Latin square design with 21-d periods. Cows were fed d iets containing either low or high soluble protein supplemented with 0 or 9 g/d of Yucca shidigera extract per cow. Dry matter intake and yi elds of milk, fat, 3.5% fat-corrected milk, and total solids were not affected by treatment. High soluble protein increased percentages of f at and total solids in milk but decreased percentages of crude protein and true protein in milk. Yucca shidigera extract did not affect perc entages or yields of milk components, ruminal NH3 N, or urea N in milk or plasma. High soluble protein tended to increase both ruminal pH an d concentrations of ruminal NH3 N. Yucca shidigera extract did not aff ect ruminal pH or NH3 N. High soluble protein increased concentrations of urea N in plasma and milk when both were measured by enzymatic ass ay, but differences in milk urea N measured by mid infrared reflectanc e spectroscopy were not significant. Neither Yucca shidigera extract n or increased ruminally undegradable protein was beneficial in this tri al when fed to cows milking 30 to 35 kg/d. Concentrations of urea N in milk as measured by mid infrared reflectance spectroscopy were higher than those measured by enzymatic assay, but measurements by mid infra red reflectance spectroscopy were not as sensitive to treatment differ ences as were measurements by enzymatic assay.