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Influence of ruminal degradable intake protein restriction on characteristics of digestion and growth performance of feedlot cattle during the late finishing phase
© May et al.; licensee BioMed Central Ltd. 2014
Received: 29 April 2014
Accepted: 16 July 2014
Published: 13 August 2014
Two trials were conducted to evaluate the influence of supplemental urea withdrawal on characteristics of digestion (Trial 1) and growth performance (Trial 2) of feedlot cattle during the last 40 days on feed. Treatments consisted of a steam-flaked corn-based finishing diet supplemented with urea to provide urea fermentation potential (UFP) of 0, 0.6, and 1.2%. In Trial 1, six Holstein steers (160 ± 10 kg) with cannulas in the rumen and proximal duodenum were used in a replicated 3 × 3 Latin square experiment. Decreasing supplemental urea decreased (linear effect, P ≤ 0.05) ruminal OM digestion. This effect was mediated by decreases (linear effect, P ≤ 0.05) in ruminal digestibility of NDF and N. Passage of non-ammonia and microbial N (MN) to the small intestine decreased (linear effect, P = 0.04) with decreasing dietary urea level. Total tract digestion of OM (linear effect, P = 0.06), NDF (linear effect, P = 0.07), N (linear effect, P = 0.04) and dietary DE (linear effect, P = 0.05) decreased with decreasing urea level. Treatment effects on total tract starch digestion, although numerically small, likewise tended (linear effect, P = 0.11) to decrease with decreasing urea level. Decreased fiber digestion accounted for 51% of the variation in OM digestion. Ruminal pH was not affected by treatments averaging 5.82. Decreasing urea level decreased (linear effect, P ≤ 0.05) ruminal N-NH and blood urea nitrogen. In Trial 2, 90 crossbred steers (468 kg ± 8), were used in a 40 d feeding trial (5 steers/pen, 6 pens/ treatment) to evaluate treatment effects on final-phase growth performance. Decreasing urea level did not affect DMI, but decreased (linear effect, P ≤ 0.03) ADG, gain efficiency, and dietary NE. It is concluded that in addition to effects on metabolizable amino acid flow to the small intestine, depriving cattle of otherwise ruminally degradable N (RDP) during the late finishing phase may negatively impact site and extent of digestion of OM, depressing ADG, gain efficiency, and dietary NE.
Because of its low cost per unit of N compared with most sources of natural protein, urea is a primary source of supplemental N in conventional steam-flaked corn-based finishing diets for feedlot cattle . In a review of nutrition consultant recommendations across 11 states in the USA, Vasconcelos and Galyean  observed that on average, flaked corn-based finishing diets contained 13.5% CP with 1.2% of supplemental urea (approximately 64% DIP). Although dietary formulation in this manner is expected to meet urea fermentation potential (UFP) for optimal microbial growth, it may exceed protein requirements for cattle growth, particularly during the late finishing phase. Preston  proposed the feasibility of restricting protein supplementation during the late finishing phase as a means of minimizing N excess and associated environmental impact [1, 4] without detrimentally affecting cattle performance. However, the impact of this practice on digestive function and cattle growth-performance has received limited research attention. The aim of this study was to evaluate the influence of UFP for optimal microbial growth on characteristics of digestion and growth performance of feedlot cattle during the late finishing phase.
All procedures involving animal care and management were in accordance with and approved by the University of California, Davis, Animal Use and Care Committee.
Diet composition of experiment 1 and 2 1
Urea fermentation potential
Ingredient (g/kg of DM)
Steam flaked corn
Trace mineral salt2
Nutrient composition (DM basis)4
RDP (g/kg of CP)
where: Yijk is the response variable, μ is the common experimental effect, Rl is the replicated effect, Si is the steer effect within replicate, Pj is the period effect within replicate, Tk is the treatment effect and Eijk is the residual error. Treatment effects were tested using the following contrasts: 1) linear effect of the urea level, and 2) quadratic effect of the urea level, which were determined according to SAS (SAS Inst., Inc., Cary, NC; Version 9.1).
Ninety crossbred steers with an average initial weight of 468 ± 8 kg were used in a 40 d finishing trial to evaluate the treatment effects on growth performance. Steers had a purchase weight of 214 ± 14 kg and had been on feed 197 d before initiation of the study. Steers had been implanted with Synovex-S (Zoetis, Florham Park, NJ) upon arrival into the feedlot and with Revalor-S (Merck Animal Health, Summit, NJ) on d 98. Ten d prior to initiation of the study steers were weighed, reimplanted with Revalor-S, blocked by weight and randomly allotted within weight groupings to 18 pens (5 steers/pen). Pens were 43 m2, with 22 m2 of overhead shade, automatic waterers, and 2.4 m long fence-line feed bunks. Dietary treatments were the same as those used in Experiment 1. All steers received the UFP-0 diet for 10 d prior to initiation of the trial. Diets were prepared at weekly intervals and stored in plywood boxes located in front of each pen. Steers were allowed free access to dietary treatments. Fresh feed was provided twice daily. Individual steers were weighed upon initiation and completion of the trial. In the calculation of steer performance live weights were reduce 4% to adjust for digestive tract fill. Estimates of steer performance were based on pen means. Net energy values for each diet were calculated from estimates of energy gain (EG, Mcal/d) based on growth-performance; EG = 0.0557 BW0.75 (ADG1.097), where EG is the daily energy deposited (Mcal/d), BW is the mean shrunk body weight (full weight × 0.96) and maintenance energy expended (EM, Mcal/d); EM = 0.077 BW0.75. Dietary NEg was derived from NEm by the equation: NEg = 0.877 NEm - 0.41 . Dry matter intake is related to energy requirements and dietary NEm according to the equation: DMI = EG / NEg), and can be resolved for estimation of dietary NE by means of the quadratic formula: , where x = NEm, a = -0.877 DMI, b = 0.877 EM + 0.41 DMI + EG, and c = -0.41 EM .
All steers were harvested on the same day. Each carcass was weighed at time of slaughter to determine dressing percentage . Performance (gain, gain efficiency, and dietary energetics) and carcass data were analyzed as a randomized complete block design; the experimental unit was the pen. The MIXED procedure of SAS  was used to analyze the variables. The fixed effect consisted of treatment, and pen was the random component. Treatments effects were tested using the following contrasts: 1) linear effect of the urea level, and 2) quadratic effect of the urea level, which were determined according to SAS .
Results and discussion
Influence of dietary treatments on characteristics of digestion
Urea fermentation potential
P - value
Flow to the duodenum (g/d)
Ruminal digestibility, %
Fecal excretion (g/d)
Postruminal digestibility (% of flow to duodenum)
Total tract digestibility (% of intake)
Passage of non-ammonia N to the small intestine decreased (linear effect, P = 0.04) with decreasing dietary urea level. This effect was due to decreased (linear effect, P = 0.04) MN synthesis. Taking into consideration energy intake alone, predicted flow of MN to the small intestine was 48g/d (, Level 1). Accordingly, with decreasing urea level, the observed flow of MN to the small intestine was 85, 73, and 65% of predicted flow for UFP-0, UFP-0.6, and UFP-1.2, respectively. This decline in net synthesis is consistent with  who observed that MN flow to the small intestine declines with decreasing DIP below 100 g/kg of total tract digestible OM. For the present study, DIP averaged 95, 81, and 61g/kg total tract digestible OM for UFP-0, UFP-0.6, and UFP-1.2, respectively. Thus, it is apparent that as DIP intake drops below 95 g/kg digestible OM there is not sufficient compensation in ruminal N recycling to maintain microbial growth, and as microbial growth declines, likewise, ruminal OM digestion declines.
There were no treatment effects (P = 0.20) on passage of feed N to the small intestine. Notwithstanding decreased non-ammonia N flow to the small intestine with decreasing urea level, ruminal N efficiency (non-ammonia N flow to the small intestine as a fraction of N intake) increased (linear P < 0.05), reflecting increased contribution of recycled N into microbial protein synthesis, consistent with the observation that ruminal N flux increases inversely with dietary N concentration . Observed DIP (Table 2) averaged 103% of expected based on tabular values (; Table 1) for the three dietary treatments.
Total tract digestion of OM (linear effect, P = 0.06), NDF (linear effect, P = 0.07), N (linear effect, P = 0.04) and dietary DE (linear effect, P = 0.05) decreased with decreasing urea level. Treatment effects on total tract starch digestion, although numerically small, likewise tended (linear effect, P = 0.11) to decrease with decreasing urea level. Decreased fiber digestion accounted for 51% of the variation in OM digestion. In a previous study involving steam-flaked corn-based finishing diets in which urea was the sole source of supplemental N , increasing urea level from 1.0 to 1.6% of the steam-flaked corn in the diet (an upper level similar to that of the present study; Table 1) likewise enhanced total tract OM and fiber digestion. In contrast Zinn and Shen  observed removal of urea from a steam-flaked corn-based growing-finishing diet markedly depressed ruminal OM digestion and flow of MN to the small intestine but did not affect total tract OM digestion. Treatment effects on apparent N digestion were largely a function of the N content of the diet brought about by changes in dietary urea level .
Treatment effects on ruminal pH, VFA molar proportions and BUN
Urea fermentation potential
P – value
Ruminal N-NH (mg/dL)
Total VFA (mM)
Ruminal VFA (mol/100 mol)
Decreasing urea level decreased (linear effect, P < 0.01) ruminal N-NH. The N-NH concentration has been reported to increase immediately after feeding for 2 to 3 h [28, 29]. Satter and Roffler  observed a close relationship (R2 = 0.92) between the level of dietary CP and ruminal N-NH concentration at given dietary TDN. Likewise, in the present study dietary CP explained 88% of the variation ruminal N-NH concentration. Blood urea nitrogen (BUN) concentration 4 h postprandium also decreased (linear effect, P < 0.01) with decreasing urea supplementation. Blood urea nitrogen is also closely associated dietary CP and ruminal N-NH concentrations [31, 32]. Consistent with Zinn et al., decreasing urea level increased ruminal acetate:propionate molar ratio (linear effect, P = 0.05), and estimated methane production (mol/mol glucose equivalent fermented; linear effect, P = 0.04).
Treatment effects on growth performance and carcass weight of feedlot steers
Urea fermentation potential
Days on test
Live weight (kg)1
Diet NE (Mcal/kg)
Treatment effects on metabolizable protein and amino acid supply 1 versus requirements 2
Urea fermentation potential
Metabolizable protein, g/d
Metabolizable methionine, g/d
Metabolizable lysine, g/d
It is concluded that in addition to effects on net protein flow to the small intestine, depriving cattle of otherwise RDP during the late finishing phase may negatively impact site and extent of OM digestion, depressing ADG, gain efficiency, and dietary NE.
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