Six 825 lb heifers were fed prairie hay and urea (45 and 90 g urea/animal/day) in two 3 x 3 Latin squares; three Zn levels were assigned within each square, with zinc chloride added at either 30 (ZN1- dietary requirement), 250 (ZN2) or 470 (ZN3) ppm of Zn in diet DM. The urea plus mineral supplements were dosed via rumen cannula once daily. After 7 d of adaptation, the rumen was sampled frequently on day 8; intake of prairie hay was measured on days 9 to 16. Responses in ruminal pH and NH₃ to urea and Zn levels differed with time. In general, pH was lower during the 12 to 24 h period than during the first 2 to 6 h period (6.68 vs 6.98). Concentrations of NH₃, highest for all treatments at 2 h (56, 43 and 35 mg/dL for ZN1, ZN2 and ZN3), were decreased by added Zn. Such a reduction may diminish the risk of urea toxicity. Rumen NH₃ concentrations decreased over time in a similar fashion for all Zn treatments. Ammonia concentrations over time differed between ZN1 vs ZN3. If 5 mg/dL is accepted as the minimum concentration needed for optimal rumen fermentation rate and microbial activity, values fell below this level sooner with ZN1 than ZN3 (8.9 vs 13.4 h). Digestible dry matter intake tended to be lower at the highest Zn level (ZN1= 5.64, ZN2= 4.98 and ZN3= 4.45 lb/animal/daily, respectively). The longer time period at higher NH₃ concentration should favor Zn supplementation.

(Key Words: Urea Utilization, Zinc, Cattle.)

Urea is an economical protein source for ruminants, and widely used because of its convenience, availability, and low cost. Although urea is frequently used in high concentrate feedlot rations, its potential should be even greater with poor quality, low protein diets. Urea provides NH₃ to the rumen that can be used for microbial protein synthesis. Increasing microbial protein yield in the rumen should increase digestibility and feed intake.

When urea is converted to NH₃ into the rumen too rapidly, incorporation of NH₃ into microbial cells is inefficient; excess of NH₃ is absorbed, converted into urea by the liver, and excreted. Excess NH₃ in the blood can cause toxicity. A shortage of carbon skeletons, such as in low quality forages, may also limit microbial protein synthesis.

By avoiding spikes of NH₃ in the rumen reducing the rate of ammonia release may improve the usefulness of urea. Several minerals can influence NH₃ production rate. High amounts of Zn or Zn plus Mn have improved urea utilization, N balance, and weight gain by sheep (Rodriguez et al., 1995). Additionally, supplementing with 860 ppm Zn significantly decreased rate of NH₃ production (Arelovich et al., 1997). Our objective was to assess the effect of supplemental Zn on ruminal metabolism and digestion of prairie hay.

Six ruminally cannulated heifers with an average weight of 825 lb were fed prairie hay (PHAY) and urea (45 or 90 g urea/animal/daily) in two 3 x 3 Latin squares. Medium quality PHAY, fed at 10.7 lb of DM split in two daily meals, was supplemented with either 30 (ZN1), 250 (ZN2) or 470 (ZN3) ppm dietary Zn from Zn chloride; additionally, Mn chloride was included in all diets to provide 40 ppm of Mn. These levels were designed to meet the Zn requirement (ZN1), provide a level estimated to be the maximum tolerated level (ZN3), and one intermediate level (ZN2). Ruminal pH and NH₃ concentrations, DM intake (DMI), DM digestibility (DMD) and digestible DMI (DDMI) were determined.

The pH of ruminal fluid was measured immediately after sampling. A 100 ml subsample was acidified with sulfuric acid and frozen. When thawed later, it was centrifuged for 10 min at 16,000 x g and analyzed for NH₃. Prairie hay, orts, and fecal samples were dried and analyzed for dry matter and ash.

Means were compared statistically using a T test; orthogonal contrasts were used to compare treatment means for the early (2 to 6 h) vs the late (12 to 24 h) periods after urea dosing. Regression of NH₃ concentration against time was also employed to test treatment effects.

Neither urea nor Zn levels significantly affected DMI or DMD (Table 1), although DDMI at the highest Zn level tended to be reduced as a result of slightly reduced digestibility. At high Zn concentrations, activities of rumen microbes may be reduced. In a previous study (Arelovich et al., 1997) similar effects were detected. DMI by animals were given free choice access to feed averaged 10.9 lb or 1.32% of the average body weight.

Response of rumen pH and NH₃ to Zn levels differed with time (Table 2), with an interaction between Zn and time in both pH (P=.049) and NH₃ (P=.066). The pH was higher during the early (2 to 6 h) than the later (12 to 24 h) period (6.98 vs 6.68; P=.0001) after dosing urea. This pH drop probably did not affect the rumen environment enough to decrease in cellulose and fiber digestion. The high pH values probably reflect the basic nature of NH₃ liberated from urea. Higher concentrations of NH₃, early after the supplement was fed, were likely followed by greater volatile fatty acid production.

Concentrations of NH₃ were decreased (P=.0001) by added Zn 2 h after feeding (56, 43 and 35 mg/dL for ZN1, ZN2 and ZN3, respectively). Rumen NH₃ concentrations decreased similarly for all Zn treatments over time being higher from 2 to 6 h than 12 to 24 h (P=.0001).

The Zn x time interaction was best described by a cubic regression as shown in Figure 1. Regression analysis revealed a significant difference in the NH₃ concentration pattern between ZN1 and ZN3 (P=.039). A minimum of 5 mg/dl is suggested to be needed for optimal activity of ruminal bacteria. Concentrations fell below this level sooner with ZN1 than ZN3 (8.9 vs 13.4 h). These results favored Zn supplements. Additionally, as shown in Figure 1, NH₃ concentration at 2 h was reduced by Zn addition. Such changes should improve efficiency of urea utilization and decrease the risk of toxicity among ruminants fed supplements containing a large amount of urea.

Arelovich, H.M. et al. 1997. Okla. Agr. Exp. Sta. Res. Rep. P-958:103.

Rodriguez, B.T. et al. 1995. Ann. Zootech. 44 (Suppl.1):229.

Appreciation is expressed to Mr. Steve Welty for the care of experimental animals.