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Influence of dietary urea level on digestive function and growth performance of cattle fed steam-flaked barley-based finishing diets

Desert Research and Extension Center, University of California, El Centro 92243

	Abstract

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Four Holstein steers (282 kg) with cannulas in the rumen and proximal duodenum were used in a 4 x 4 Latin square experiment to evaluate the influence of dietary urea level (0, 0.4, 0.8, and 1.2%, DM basis) in a steam-flaked barley-based finishing diet on digestive function. There were no treatment effects (P > 0.20) on ruminal digestion of OM and ADF. Increasing dietary urea level increased (linear, P < 0.01) ruminal starch digestion. Ruminal degradability of protein in the basal diet (no supplemental urea) was 60%. Increasing dietary urea level did not increase (P > 0.20) ruminal microbial protein synthesis or nonammonia N flow to the small intestine. There were no treatment effects (P > 0.20) on total-tract ADF digestion. Total tract digestion of OM (quadratic, P < 0.01) and starch (linear, P < 0.05) increased slightly with increasing urea level. Urea supplementation increased (linear, P < 0.01) ruminal pH 1 h after feeding; however, by 3 h after feeding, ruminal pH was lower (cubic, P < 0.05) with urea-supplemented diets. Urea supplementation did not affect (P > 0.20) ruminal molar proportions of acetate and propionate. One hundred twenty crossbred steers (252 kg; approximately 25% Brahman breeding) were used in an 84-d feeding trial (five pens per treatment) to evaluate treatment effects on growth performance. Daily weight gain increased (linear, P = 0.01) with increasing urea level, tending to be maximal (1.53 kg/d; quadratic, P = 0.13) at the 0.8% level of urea supplementation. Improvements in ADG were due to treatment effects (linear, P < 0.01) on DMI. Urea supplementation did not affect (P > 0.20) the NE value of the diet for maintenance and gain. Observed dietary NE values, based on growth performance, were in close agreement with expected based on tabular values for individual feed ingredients, averaging 100.4%. We conclude that with steam-flaked barely-based finishing diets, ruminal and total-tract digestion of OM and ruminal microbial protein synthesis may not be increased by urea supplementation. In contrast, ADG was optimized by dietary inclusion of 0.8% urea. Urea supplementation may not enhance the net energy value of steam-flaked barely-based finishing diets when degradable intake protein is greater than 85% of microbial protein synthesis.

Key Words: Barley • Cattle • Digestion • Performance • Urea

	Introduction

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Based on early reviews, it was proposed that supplemental nonprotein nitrogen (NPN) could be effectively utilized by cattle provided it consisted of not more than one-third of the total N or 1% of total diet DM (Stangel, 1963; Chalupa, 1968). Later, Burroughs (1975) observed that the upper limits for NPN utilization could be factorialized on the basis of fermentation potential. The urea fermentation potential of a diet is derived from measures of net microbial protein synthesis and ruminal degradation of dietary protein (DIP). Burroughs et al (1975) proposed that microbial N flow to the small intestine was equivalent to 0.0166 TDN. The NRC (1996; Level 1) estimates net ruminal microbial N flow to the small intestine as:

where TDN is expressed in kg/d, and effective neutral detergent fiber (eNDF) as a percentage of DMI. In either case, it is assumed that amount of urea that should be added to the diet to optimize ruminal microbial growth is equivalent to net microbial protein synthesis minus the DIP content of the basal diet divided by 2.8. This approach, although conceptually appealing, has limited empirical support. Indeed, there is considerable evidence (Lofgreen et al., 1968; Zinn et al., 1994; Milton et al., 1997) that levels of urea supplementation in excess of that required to optimize microbial protein synthesis may enhance growth performance of feedlot cattle. The basis for this effect may be due to the alkalizing effects of urea as it is hydrolyzed within the rumen to form ammonium carbonate. The objective of this study was to evaluate the influence of level of urea supplementation on digestive function, and growth performance of cattle fed a steam-flaked barley-based finishing diet.

	Experimental Procedures

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Trial 1
Four Holstein steers (282 kg) with cannulas in the rumen and proximal duodenum (Zinn and Plascencia, 1992) were used in a 4 x 4 Latin square experiment. Treatments consisted of a steam-rolled barley-based finishing diet supplemented with 0, 0.4, 0.8, and 1.2% urea. The experimental diet compositions are shown in Table 1. Chromic oxide (0.4%, DM basis) was included in the diets as a digesta marker. Dry matter intake was restricted to 6 kg/d and fed in equal portions at 0800 and 2000 daily. The four experimental periods consisted of a 10-d diet adjustment period followed by a 4-d collection period. During the collection period, duodenal and fecal samples were taken from all steers twice daily as follows: d 1, 0750 and 1350; d 2, 0900 and 1500; d 3, 1050 and 1650; and d 4, 1200 and 1800. Individual samples consisted of approximately 700 mL of duodenal chyme and 200 g (wet basis) of fecal material. Samples from each steer and within each collection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer at 1, 2, 3, 4, and 5 h after feeding via the ruminal cannula. Ruminal fluid pH was determined on fresh samples. Samples were strained through four layers of cheesecloth. Two milliliters of freshly prepared 25% (wt/vol) meta-phosphoric acid was added to 8 mL of strained ruminal fluid. Samples were then centrifuged (17,000 x g for 10 min) and supernatant fluid stored at -20°C for VFA analysis. Upon completion of the trial, ruminal fluid was obtained from all steers and composited for isolation of ruminal bacteria via differential centrifugation (Bergen et al., 1968). Samples were subjected to all or part of the following analysis: DM (oven drying at 105°C until no further weight loss); ash, Kjeldahl N, ammonia N (AOAC, 1984); ADF (Goering and Van Soest, 1970); purines (Zinn and Owens, 1986); chromic oxide (Hill and Anderson, 1958), starch (Zinn, 1990), and VFA concentrations of ruminal fluid (gas chromatography; Zinn, 1988). Duodenal flow and fecal excretion of DM were calculated based on marker ratio using chromic oxide. The amount of microbial organic matter (MOM) and microbial N (MN) leaving the abomasum was calculated using purines as a microbial marker (Zinn and Owens, 1986). Organic matter fermented in the rumen was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and MOM reaching the duodenum. Feed N escape to the small intestine was considered equal to total N leaving the abomasum minus ammonia N and MN, and thus includes any endogenous contributions. The trial data were analyzed based on a 4 x 4 Latin square experimental design according to the following statistical model:

where A_i is steer, P_j is period, T_k is treatment and E_ijk is residual error. Treatment effects were tested for linear, quadratic and cubic components by means of orthogonal polynomials (Hicks, 1973).

Trial 2
One hundred twenty crossbred steer calves (approximately 25% Brahman breeding with the remainder represented by Hereford, Angus, Shorthorn, and Charolais breeds in various proportions) with an average initial weight of 252 kg were used in an 84-d feeding trial to evaluate the treatment effects on growth performance. Steers were blocked by weight and allotted randomly within weight groupings to 20 pens (six steers/pen). Pens were 43 m², with 22-m² overhead shade, automatic waterers, and 2.4-m fence-line feed bunks. During the course of the trial (December through February), minimum and maximum daily air temperatures averaged 3.9 and 21.1°C, respectively, Relative humidity averaged 57.7%. Precipitation totaled 1.5 cm. Dietary treatments were the same as those used in Trial 1. At the start of the trial, steers were implanted (Synovex-S, 20 mg of estradiol benzoate plus 200 mg of progesterone; Fort Dodge Animal Health, Overland Park, KS). Diets were prepared at approximately 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 (unshrunk) upon initiation and completion of the trial. In the calculation of steer performance, live weights were reduced 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 (NRC, 1984; EG_{medium frame steer calf} = [0.0557BW^0.75]ADG^1.097, where BW is the mean shrunk body weight [full weight x 0.96]), and maintenance energy expended (Mcal/d, EM; EM = 0.077BW^0.75; NRC, 1984), using the quadratic formula, where x = dietary NE_m (Mcal/kg), a = -0.877DMI (kg/d), b = 0.877EM + 0.41DMI + EG, c = -0.41EM, and dietary NE_g = 0.877NE_m - 0.41 (Zinn and Shen, 1998). The trial data were analyzed based on a randomized complete block experimental design according to the following statistical model:

where B_i is blocks, T_j is treatment, and E_ij is residual error, using pen means as the experimental units. Treatment effects were tested for linear, quadratic, and cubic components by means of orthogonal polynomials (Hicks, 1973).

	Results and Discussion

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The influence of dietary urea level on ruminal and total-tract digestion of a steam-flaked barley-based finishing diet is shown in Table 2. There were no treatment effects (P = 0.72) on ruminal digestion of OM, averaging 64% (80% of total-tract OM digestion). Likewise, there were no treatment effects (P = 0.74) on ruminal digestion of ADF, averaging 32%.

There were no treatment effects (P = 0.49) on flow of nonammonia N to the small intestine. Apparent ruminal degradation of dietry nonurea CP averaged 60%. Adjusting for endogenous contributions to N flow to the small intestine (0.195 g/kg BW^0.75; Ørskov et al., 1986), true ruminal degradation of dietary nonurea CP was 72.5%. The steam-flaked barley used in this trial contained 11.8% CP, comprising 85.1% of the total CP content of the basal diet. Given that the ruminal degradability of the other protein-containing ingredients of the basal diet (alfalfa, sudangrass, and cane molasses) were consistent with NRC (1996), then by difference, the ruminal degradation of steam-flaked barley protein in this trial was 72%. Likewise, Zinn et al. (1996), using a replacement technique (steam-flaked barley replaced steam-flaked corn), observed that true ruminal degradation of barley protein was 70%. The tabular value (NRC, 1996) for ruminal degradability of barley protein is 67%.

With the basal diet, flow of microbial N to the small intestine was 86 g/d, whereas ruminal degradable N was 73 g/d, indicative of a positive urea fermentation potential. However, increasing dietary urea level did not enhance (P = 0.42) flow of ruminal microbial N to the small intestine. Indeed, microbial efficiency (g microbial N/kg OM truly fermented) decreased slightly (linear, P < 0.05) with increasing urea level.

Burroughs et al. (1975) proposed that microbial N flow to the small intestine was equivalent to 0.0166 TDN. The tabular (NRC, 1996) TDN value of the basal diet used in this trial was 85.4%. Thus, the predicted flow of microbial N to the small intestine would be 84 g (0.0166 x 0.854 x 5,948; where 5,948 is the DMI; Table 2), in very close agreement with observed (86 g; Table 2). Burroughs et al. (1975) further proposed that amount of DIP necessary to optimize microbial growth was equivalent to the net microbial protein synthesis. Accordingly, the urea fermentation potential of the diet (percentage of additional urea that may be added to the diet in order to optimize microbial growth) would be equivalent to: (0.104TDN - DIP)/2.8, where TDN is expressed as a percentage, and DIP is expressed as the percentage of ruminally degradable protein in the basal diet before urea supplementation (7.69%). Thus, it would be expected that the dietary urea level necessary for optimizing ruminal microbial protein synthesis is 0.43%.

The NRC (1996; Level 1) estimates net ruminal microbial CP synthesis (g/d) as:

where TDN is expressed in kg/d, and eNDF as a percentage of DM intake. Accordingly, the expected microbial N flow to the small intestine for steers fed the basal diet would be 88 g. Like Burroughs et al. (1975), the NRC (1996) also assumes that the DIP required for optimizing net microbial N flow to the small intestine is equivalent to the expected microbial CP flow to the small intestine. Thus, the expected urea fermentation potential of the basal diet was 0.56% based on NRC (1996):

The overestimation of dietary urea fermentation potential by these two systems may be due largely to their failure to take into consideration recycled N. Kennedy and Milligan (1980) observed that N recycled to the rumen (RN, percentage of N intake) was a predictable (R² = 0.97) function of dietary CP level (DCP, %):

Accordingly, N recycled to the rumen of steers fed the basal diet was 31% of intake, or 32 g/d. The combination of recycled N plus DIP was 105 g/d, 122% of net microbial N synthesis (86 g; Table 2).

Zinn and Shen (1998) observed that microbial protein synthesis was maximized when DIP was greater than 75% of microbial N flow to the small intestine (equivalent to 100 g of DIP/kg of total-tract digestible OM). For the present study (Trial 1), this amounts to 67 g of degradable feed N.

Consistent with previous studies (Zinn et al., 1994), total-tract OM digestion increased slightly (2%; quadratic, P < 0.01) with urea supplementation. There were no treatment effects (P = 0.50) on total-tract ADF digestion, averaging 49.0%. Total-tract starch digestion increased slightly (linear, P = 0.05) with dietary urea level. Although consistent with other studies evaluating steam-flaked barley (Zinn, 1993; Zinn et al., 1996; Zinn and Barajas, 1997), starch digestion was nearly complete across treatments, averaging 99.4%.

As expected, apparent total-tract N digestion increased linearly (P < 0.01) with increasing dietary urea level. This increase is due in part to the digestibility of urea, and in part to the increasing dietary CP concentration with increasing urea level (Holter and Reid, 1959).

Treatment effects on ruminal pH, total VFA, VFA molar proportions, and estimated methane production during the first 5 h after feeding are shown in Table 3. Urea supplementation increased (linear effect, P < 0.01) ruminal pH 1 h after feeding. This initial increase in ruminal pH is consistent with the ruminal alkalizing effects of urea. Urea is a carbonyldiamide (H₂NCONH₂). Each mole of urea releases two moles of ammonia upon hydrolysis by urease, a process that occurs rapidly within the rumen. As a weak base with a negative logarithm of equilibrium constant of 8.8 in ruminal fluid, less than 0.2% of the ammonia released by urease will be in a nonionized form. Thus, in addition to its role as a N source for ruminal microbes, dietary urea will also have an appreciable alkalizing effect on ruminal pH. However, as also noted for sodium bicarbonate and magnesium oxide supplementation of finishing diets (Montano et al., 1999), this response was transitory; although by 3 h after feeding, ruminal pH was lower (cubic, P < 0.05) with urea-supplemented diets. This trend for decreased ruminal pH with urea supplementation continued (linear, P < 0.10) throughout the remainder of the sampling period (4 and 5 h after feeding). The basis for this effect is not certain. As was expected, most of the variation in ruminal pH can be explained by ruminal VFA concentration (pH = 11.88 - 0.082VFA + 0.000268VFA²; R² = 86.6). Although we did not measure the interaction of urea supplementation on rate of feed intake, changes in the pattern of feed consumption can have appreciable effects on ruminal VFA concentrations and pH (Montano et al., 2001).

Treatment effects on growth performance of feedlot steers (Trial 2) are shown in Table 4. Daily weight gain increased (linear, P = 0.01) with increasing dietary urea level, tending to be maximal (1.53 kg/d; quadratic, P = 0.13) at the 0.8% level of urea supplementation. Improvements in ADG were due to treatment effects (linear, P < 0.01) on DMI. Consistent with measures of OM digestion (Table 2), urea supplementation did not affect (P = 0.47) estimates of dietary NE based on growth performance. Indeed, observed dietary NE values, based on growth performance, were in close agreement (100.4%) with expected based on tabular values for individual feed ingredients (Table 1). Although the basis for improved DMI with increasing dietary urea level is not certain, the improved ADG is consistent with earlier studies (Lofgreen et al., 1968; Zinn et al., 1994; Healy et al., 1995) demonstrating that feedlot cattle growth-performance may be enhanced by levels of urea supplementation in excess of that required to optimize microbial protein synthesis.

	Implications

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	Footnotes

 
² Current address: Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Sinaloa, Culiacán, Sin. (México).

³ Current address: Centro de Investigación en Ciencias Veterinarias, Universidad Autónoma de Baja California, Mexicali, B.C., (México).

¹ Correspondence—phone: 760-356-3068; fax: 760-356-3068; E-mail: razinn{at}ucdavis.edu.

Received for publication December 10, 2002. Accepted for publication June 6, 2003.

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