Effects of encapsulated nitrate on toxicity, feed intake and feed consumption rates in beef cattle

Abstract Text: Slow-release encapsulated nitrate [EN; 66.9% nitrate in DM; GRASP Ind. & Com. LTDA, Paraná, Brazil] was investigated for its effects on toxicity threshold, feed intake, and feed consumption rates in 2 experiments. In Exp. 1, 5 beef heifers were fed once daily a diet (55:45 forage:concentrate) at 75% of ad libitum intake. The proportion of EN in the diet was increased by 1% every 4 days to 1.0, 2.0, 2.9, 3.9, 4.8, and 5.8% of dietary DM (10.3 to 15.6% CP at 0 to 5.8% EN). In Exp. 2, 8 beef heifers were used in a replicated 4 × 4 Latin square design and fed ad libitum once daily diets (iso-nitrogenous, 12.7% CP; 55:45 forage:concentrate) containing 0, 1, 2, and 3% EN (DM basis) using a 21-d stepwise adaptation. In Exp. 1 with restrictive feeding, a heifer was removed due to rapid feed consumption causing nitrate-poisoning at 3% EN and another due to refusal to eat the 2% EN diet. Comparing 0% with 5.8% EN, feed consumption from 0 to 3 h after feeding was decreased (70.3 to 48.6% of total; P = 0.03), that from 12 to 24 h was increased (0.6 to 22.6%; P = 0.01), and feed consumed over 24 h was decreased (100 to 92.3%; P = 0.01). Animals showed negligible blood methemoglobin levels (< 1%; MetHb, % of hemoglobin) at 1% EN. However, MetHb levels were greater (avg. 8.6 vs. 3.3% and max. 23.6 vs. 13.6% at 3 h) at 2.0 and 2.9% EN than at 3.9% or more EN because of sorting of the concentrates containing higher levels of EN. The MetHb level peaked 3 h after feeding for all EN levels and the magnitude of the peak was dependent on the amount of feed consumed from 0 to 3 h. In Exp. 2 with ad libitum feeding, feed consumption rates were not different (41.8% of total from 0 to 3 h) among EN levels. Although MetHb levels increased (P < 0.01) with increasing EN level, the magnitude was lower for Exp. 2 compared with Exp. 1 (avg. 2.8 vs. 8.4%; max. 7.2 vs. 23.6% at 3% EN). Sorting of the EN diets was not observed in Exp. 2. In conclusion, MetHb responses to EN levels were dependent on feed consumption rates after feeding. Offering a diet containing EN for ad libitum intake minimized risks of nitrate toxicity.

Keywords: Encapsulated nitrate, Methemoglobin, beef cattle