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 nitratefeeding
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