| Abstrakt: |
The objective of the study was to investigate the effects of feed consumption rate on potential toxicity, rumen fermentation, and eating behavior when beef heifers were fed a diet supplemented with nitrate (NI). Twelve ruminally cannulated heifers (827 ± 65.5 kg BW) were used in a randomized complete block design. The experiment consisted of 10-d adaptation, 8-d urea-feeding, and 3-d nitrate-feeding periods. All heifers were fed a diet supplemented with urea (UR) during the adaptation and urea-feeding periods, whereas the NI diet (1.09% NO3- in dietary DM) was fed during the nitrate-feeding period. After adaptation, heifers were randomly assigned to ad libitum or restrictive feeding (about 80% of ad libitum intake) for the urea- and nitrate-feeding periods. Ad libitum DMI decreased (14.1 vs. 15.1 kg/d; P < 0.01) when heifers were fed the NI diet compared with the UR diet. The amount of feed consumed increased (P < 0.01) at 0 to 3 h and decreased (P ≤ 0.03) at 3 to 24 h for restrictive vs. ad libitum feeding of both the UR and NI diets. Compared to the UR diet, the NI diet decreased (P < 0.01) feed consumption at 0 to 3 h and increased (P < 0.02) feed consumption at 3 to 24 h (except feed consumption at 9 to 12 h; P = 0.90), indicating nitrate feeding changed the consumption pattern (a more even distribution of feed intake over the day). The increased feed consumption from 0 to 3 h after feeding the NI diet restrictively vs. ad libitum numerically decreased (P = 0.11) rumen pH and numerically or significantly increased (P = 0.01 to 0.28) rumen ammonia, NO3-, and NO2-; blood methemoglobin; and plasma NO3- and NO2- at 3 h. Regression analysis indicated that increased feed consumption (0 to 3 h) exponentially elevated (P < 0.01; R² = 0.75) blood methemoglobin, and plasma NO3- + NO2- among other rumen and blood variables had the greatest correlation (sigmoid response; P < 0.01, R² = 0.47) with feed consumption (0 to 3 h). Particle size distribution of orts was partially altered (P = 0.02 to 0.40) when the NI diet was fed compared with the UR diet. During the nitrate-feeding period, the nitrate content of orts on d 2 and 3 was greater (P = 0.02) than that on d 1. In conclusion, the increased consumption rate of a diet supplemented with nitrate was an important factor influencing risk of nitrate toxicity based on blood methemoglobin and plasma NO3-. In addition, the pattern of daily feed consumption was altered by nitrate (creating a "nibbling" pattern of eating) in beef heifers. [ABSTRACT FROM AUTHOR] |
