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Soybean meal replaced by slow release urea in finishing diets for beef cattle
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Soybean meal replaced by slow release urea in finishing diets for beef cattle P.D.B. Benedeti a,n, P.V.R. Paulino b, M.I. Marcondes a, S.C. Valadares Filho a, T.S. Martins a, E.F. Lisboa a, L.H.P. Silva a, C.R.V. Teixeira a, M.S. Duarte a a b
Department of Animal Science, Universidade Federal de Viçosa, Viçosa 36571-000, Minas Gerais, Brazil Cargill Animal Nutrition/Nutron
a r t i c l e i n f o
abstract
Article history: Received 20 August 2013 Received in revised form 12 March 2014 Accepted 24 April 2014
Eight crossbred steers (average body weight of 418 kg) fitted with ruminal and abomasal cannula were used to evaluate the effects of replacing soybean meal (SBM) with slowrelease urea (SRU) in beef cattle diets containing two concentrate levels. The experimental design included two 4 4 Latin squares, which were run simultaneously. Each Latin square received one level of concentrate [400 or 800 g/kg on a dry matter (DM) basis]. Within each Latin square, the four replacement levels of soybean meal protein with slowrelease urea were applied to the animals (0%, 33%, 66% and 100% of substitution on N basis). The DM intake as well as organic matter (OM) intake and crude protein (CP) intake decreased linearly (P o 0.05) as SBM was replaced with SRU. Ruminal digestibility coefficient of OM tended to be greater (P ¼ 0.074) for the 40 % concentrate diet. DM and OM passage rate (kp) were greater (P o 0.05) on the 80% concentrate diet. A cubic effect (P o 0.10) of SBM replacement with SRU on ruminal ammonia (NH3–N) concentration in relation to time was detected. A quadratic effect on pH was observed (P o0.10) when replacing SBM with SRU. Nitrogen intake, nitrogen excreted in the feces, nitrogen balance and efficiency of nitrogen use decreased linearly (P o0.10) as SRU increased in the diet, whereas the total nitrogen excreted in urine increased linearly (P ¼ 0.007). The production of microbial nitrogen and microbial efficiency were not affected by the experimental treatments (P4 0.10). A lower intake of DM, OM, and CP was observed when cattle were fed SRU compared to SBM. However, the use of SRU did not change the digestibility and digestion rate (kd) and kp of DM, OM, CP and neutral detergent fiber corrected for ash and protein (NDFap). In summary, SRU provides higher concentrations of NH3–N throughout a day than SBM in cattle fed low concentrate diets. & 2014 Elsevier B.V. All rights reserved.
Keywords: Cannulated Feedlot Non-protein nitrogen pH Ruminal ammonia
1. Introduction High concentrate diets (HCD) have been intensively used in feedlots in Brazil as it improves intake, body weight gain and carcass weight gain in beef cattle (Keane
n
Corresponding author. Tel.: þ1 (775) 409 9653. E-mail address: [email protected] (P.D.B. Benedeti).
et al., 2006). Most of these feeding systems use high grain diets and soybean meal (SBM) as a crude protein (CP) source (Millen et al., 2009). However, due to the high costs of soybean, beef producers have been seeking for an alternative protein source that would reduce the feeding costs. As such, the use of non-protein nitrogen (NPN) in ruminant diets appears as a viable strategy to enhance ruminal microbial protein (Storm and Ørskov, 1983) and consequently reduce feeding costs.
http://dx.doi.org/10.1016/j.livsci.2014.04.027 1871-1413/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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Urea is the main NPN source used in beef cattle diets in Brazil (Millen et al., 2009). However, there are still concerns regarding its use due to rapid release of ammonia (NH3–N), which can be faster than its use by microorganisms for protein synthesis. The efficiency of protein synthesis from urea depends, among other factors, on energy availability in the rumen (Russell et al., 1992). Further, a fast release of nitrogen may cause an asynchrony of NH3–N and energy in the ruminal environment. As a consequence, excessive NH3–N can decrease animal performance and, in some cases, cause ammonia toxicity (Bartley et al., 1976; Huntington et al., 2006; Owens et al., 1980). The use of slow-release urea (SRU) may improve the synchrony of energy and NH3–N in the rumen, leading to a better efficiency of ruminal bacteria growth. Improving the NH3–N and energy synchrony in the ruminal environment using SRU would reduce use of true protein sources in beef cattle diets. Moreover, since fiber digestion may be improved as a consequence of continuous release of nitrogen in the ruminal environment (Alvarez-Almora et al., 2012), the use of SRU appears as a great strategy to enhance the efficiency of use of fibrous feed sources. Consequently, the use of SRU may reduce feeding costs without compromising animal performance. Studies have reported efficacy in reducing NH3–N release in the rumen (Highstreet et al., 2010; Huntington et al., 2006; Owens et al., 1980), ruminal fermentation (Owens et al., 1980), microbial protein synthesis (Cherdthong et al., 2011; Xin et al., 2010), pH (TaylorEdwards et al., 2009), digestibility, and intake (Cherdthong et al., 2011) when SRU replaced conventional urea. However, the consequences of replacing sources of true protein with SRU in diets with different levels of concentrate remain unclear. Therefore, this study was performed to evaluate the effects of replacing SBM with SRU in beef cattle diets containing two concentrate levels on ruminal parameters of beef cattle. 2. Materials and methods The work described was carried out in accordance with EC Directive 86/609/EEC for animal experiments. 2.1. Animals, experiment design and diets Eight Bos indicus steers, with an average body weight (BW) of 418 740 kg and 24 months of age, were used. All animals were cannulated in the rumen and abomasum (Kehls). The animals were kept in a tie stall. All animals were initially weighed, treated for elimination of internal parasites, and then fed the experimental diets during 15 days for adaptation prior to the beginning of the experiment. Cattle adaptation to the experimental diets was performed gradually from 1% of BW followed by an increase of 0.2% of BW as dry matter (DM) every 3 days until the remaining unconsumed feed reached 10% of the DM offered. After the adaptation period, the animals were submitted to a total of four experimental periods of 15 days with 7 days for animal adaptation (Storry and Sutton, 1969) and 8 days for sample and data collection. The experimental design included two 4 4 Latin squares, which were run simultaneously. Each Latin square
had one of the two concentrate levels evaluated (60:40 roughage:concentrate ratio for low concentrate diet (LCD), and 20:80 roughage:concentrate ratio for HCD. Within each Latin square, the four replacement levels of soybean meal protein with SRU (Optigens 1200 controlled-release N, Alltechs, Araucária-PR, Brazil) were applied to the animals (0%, 33%, 66% and 100% of substitution on an N basis) totaling eight treatments with four replicates each. Experimental diets were composed of corn silage, corn meal, SBM and mixture mineral. Diets were formulated to be isonitrogenous containing 120 g/kg CP on a DM basis, in order to meet the nutritional requirements of beef steers with 400 to 500 kg of BW (Valadares Filho et al., 2010). Ingredient proportion and chemical composition of the experimental diets are presented in Table 1. 2.2. Experimental procedures and sample collections Animals were fed once daily at 0700 h, allowing for up to 10% of orts. Dry matter intake was determined from day 8 to day 15. A total feces collection was performed during three consecutive days (days 8, 9 and 10 of each experimental period) to estimate digestibility of dietary constituents, as suggested by Barbosa et al. (2006), Ferreira et al. (2009), Mezzomo et al. (2011) and Paixão et al. (2007). The variation of DM intake during the fecal collection period is presented in Fig. 1. From day 8 to 11 of each experimental period, abomasal digesta was sampled at intervals of 15 h as follows: day 8, sampling at 0800 h and 2300 h; day 9, sampling at 1400 h; day 10, sampling at 0500 h and 2000 h; and day 11, sampling at 1100 h as described by Allen and Linton (2007). Samples were frozen at 80 1C, freeze-dried for 72 h, and then ground through a 1-mm screen in a Wiley Mill. At the end of the process, a composite sample was prepared for each animal in each sampling period. Dry matter flux was determined by adding an external marker (Cr2O3) in the rumen and their concentration was measured in abomasal digesta. A daily dose (15 g) of the marker was added through the ruminal cannula at 1200 h from day 3 to 10 and the chromic oxide (Cr2O3) concentration was determined as described by Savastano (1993). Rumen evacuations for determining rumen pool size and digesta kinetics were carried out on day 12, four hours after feeding and on day 14 immediately before feeding (Allen and Linton, 2007; Mezzomo et al., 2011). Rumen contents were collected into a plastic container and separated into primarily liquid and particulate fractions by filtering through screening. Solid and liquid fractions were placed in different containers, individually weighed and sub-sampled (500 g for solids, and 2 kg for the liquid fraction) for further analysis. From day 8 to 10 urine was collected over a 24 h period using funnel collectors attached to animals with a polyethylene flexible tube that transported the urine to containers containing 250 ml of a 20% H2SO4 solution (vol:vol) to avoid loss of nitrogenous compounds (Valadares et al., 1997). Microbial biomass was determined by purine bases quantification according to Ushida et al. (1985). Ruminal liquids were sampled on day 11 to determine pH and NH3–N. Approximately 50 mL of liquid samples were manually collected from the ruminal cannula at 0, 2, 4, 6,
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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Table 1 Composition and analyzed nutrient of experimental diets. Replacement of SBM by SRU (%)
Experimental diets Low concentratea
g/kg of DM Corn silage Corn meal Soybean meal SRU Mineral mixture g/kg of DM DM (g/kg) OM CP EE NDFap NFC NPN/N
High concentrateb
0
33
66
100
0
33
66
100
600 258.0 112.0 – 30
600 288.1 74.8 7.1 30
600 318.1 37.7 14.2 30
600 348.6 – 21.4 30
200 678.0 92.0 – 30
200 708.2 56.0 5.8 30
200 738.2 20.2 11.6 30
200 752.4 – 17.6 30
505.7 937.1 119.3 37.4 336.5 443.9 260.4
499.6 931.5 120.6 37.9 335.8 448.3 384.1
493.4 925.8 121.9 38.3 335.0 452.7 505.1
487.1 920.1 123.3 38.8 334.3 457.2 625.2
732.1 951.7 118.9 39.3 208.3 585.1 208.1
727.1 947.3 117.5 39.9 207.8 591.1 313.0
722.0 942.9 116.2 40.4 207.3 597.1 420.3
716.8 937.7 122.2 40.6 206.6 595.7 512.0
SRU¼ slow release urea, DM¼ dry matter, OM¼ organic matter, CP¼ crude protein, EE ¼ ether extract, NDFap ¼neutral detergent fiber corrected for ashes and protein, NFC ¼non-fiber carbohydrates, NPN/N ¼ non-protein nitrogen relation to total nitrogen a Low concentrate¼ 400 g concentrate/kg of DM b High concentrate¼ 800 g concentrate/kg of DM
Fig. 1. Average of dry matter intake during the weeks of fecal collection and replacement of soybean meal (SBM) with slow release urea (SRU) for low and high concentrate diets. Low¼400 g concentrate/kg of DM; High ¼800 g concentrate/kg of DM.
8, 10, 12, 14, 16, 18, 20 and 22 h after feeding, filtered through a triple cheesecloth layer, and pH was immediately measured by a digital pH meter (Digmed Model DM21, Digicrom, São Paulo, Brazil). After pH reading, 1 mL of a 50% H2SO4 solution (vol:vol) was added for later determination of NH3–N concentration (Chaney and Marbach, 1962). On day 15 and 4 h after feeding, blood samples were collected and immediately centrifuged at 2700 g for 20 min to obtain blood serum. 2.3. Chemical analysis and calculations Feed, feces, orts, and abomasal and ruminal samples were analyzed for DM (method 934.01; AOAC, 1990), ash (method 938.08; AOAC, 1990), CP (method 984.13; AOAC, 1990), ether extract (EE; method 920.85; AOAC, 1990), and non-protein nitrogen (NPN; Licitra et al., 1996). The organic matter (OM) was calculated as the difference between DM and ash contents. For analyzing the neutral
detergent fiber corrected for ash and protein (NDFap) concentration, the samples were treated with alpha thermo-stable amylase without sodium sulfite and corrected for residual ash (Mertens, 2002) and for residual nitrogenous compounds (Licitra et al., 1996). The levels of metabolizable energy (ME) were calculated according to NRC (2001), where ME (MJ kg 1)¼4.184 [1.01 (DCP 5.6þ DEE 9.4þDNDFap 4.2þDNFC 4.2) 0.45]. Here DCP¼digestible crude protein, DEE¼digestible ether extract, DNDFap¼digestible neutral detergent fiber corrected for ash and protein, and DNFC¼ digestible nonfiber carbohydrate. The non-fiber carbohydrates (NFC) were calculated as NFC¼100[(CP CP from ureaþUrea)þ NDFapþEEþAsh] (Detmann and Valadares Filho, 2010). Passage rate (kp) and digestion rate (kd) of dietary constituents from the data of rumen evacuation were calculated according to Allen and Linton (2007). The rate of disappearance (turnover rate) of the digesta fraction from the rumen was calculated by dividing its rate of
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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intake by its ruminal pool size [ki (rate of intake; h 1)¼ intake (kg h 1) / rumen pool (kg)]. The fractional passage rate of a homogeneous fraction was calculated by dividing the flux of the fraction passing from the rumen by its ruminal pool size [kp (rate of passage; h 1)¼passage flux (kg h 1)/rumen pool (kg)]. The digestion rate of the fraction could then be calculated by subtracting its passage rate from its turnover rate in the rumen [kd (rate of digestion; kg h 1)¼ki kp]. The average rumen pool size was determined for digesta fractions by evacuating total rumen contents, weighing and sub-sampling the contents, and analyzing the sub-sample. The ruminal coefficient of digestibility was determined using the average intake and the estimated amount of DM and diet components in the abomasum. Intestinal digestibility was calculated using the estimated amount of DM and diet components in the abomasum and the amount of DM in feces (Mezzomo et al., 2011). Urine samples were analyzed for allantoin and uric acid by a colorimetric method as described by Chen and Gomes (1992). The intestinal flow of nitrogenous compounds was calculated based on the amount of microbial purines absorbed, according to the equation proposed by Chen and Gomes (1992). The flux of microbial N abomasal digest was calculated using the purine-N/total-N ratio obtained in this experiment. The microbial efficiency was calculated as total microbial biomass/TDN intake (NRC, 2001; Valadares Filho et al., 2010). Calculation of N balance was conducted using N intake from each of the three collection days (day 8 to 10) when animal feed intake was measured, minus the amount of N excreted. Excreted N was estimated based on feces and urine N content during collection days.
analyzed using the following model: Y ijkl ¼ β0 þ β1 αi þ β2 λj þ β12 αi λj þβ3 λ2j þ β13 αi λ2j þP k þAl þeijkl ; where Yijkl is the observed measurement of the ith level of concentrate in the diet and of the jth level of substitution of SBM with SRU of the kth period and of the lth animal; I¼1, 2 (levels of concentrate in the diet), j¼1, 2, 3, 4 (levels of inclusion of SRU as a replacement of SBM), β0, β1, β2, β12, β3, β13 ¼regression parameters of the model; αi ¼effect of ith fixed qualitative factor (level of concentrate, two levels); λj ¼effect of jth level of fixed quantitative factor (replacement of SBM with SRU); Pk ¼effect of level k of random factor period; Al ¼effect of level of random factor animal; eijkl ¼ unexplained residual error, assuming eijk N (0, s2), with independent errors. Only pH and NH3–N data were analyzed as a repeated measurements design. Therefore, the model tested was similar to the model above, including linear, quadratic and cubic effects of time over pH and NH3–N. All one-way, two-way and three-way interactions were tested. Outliers were identified when the Studentized residue was greater than 2.5 or smaller than 2.5. All non-significant effects (P40.10) were removed from the model to determine the final equation. In this case, the model had two errors, one random error with mean 0 and variance s2 which is the variance between animals within treatment and equal to the covariance between repeated measurements within animals; and the general residual error with the mean 0 and variance s2, which is the variance between measurements within animals.
3. Results
2.4. Statistical analysis
3.1. Intake
For statistical analysis, all results were tested for normality (Davis et al., 1989) and they all followed normal distribution (P40.05). All statistical procedures were carried out using SAS 9.2 for Windows (Statistical Analysis System Institute, Inc., Cary, NC, USA) with α¼0.10. Intake, total and partial digestibility, dynamics of ruminal degradation, nitrogen balance and microbial protein production were
Concentrate levels did not affect (P ¼0.283) DMI as a function of BW, which were 19.2 72.9 and 21.673.7 g/kg for LCD and HCD, respectively (Table 2). However, a linear decrease of DM intake as a function of BW was detected (P¼0.046) due to inclusion of SRU in the diets. The inclusion of SRU also decreased linearly the DM, CP, and OM intake. Concentrate level affected (Po0.10) NFC, and
Table 2 Effect of replacing soybean meal (SBM) with slow release urea (SRU) and concentrate levels on daily intake of dietary constituents in beef cattle.
DM, kg OM, kg CP, kg EE, kg NDFap, kg NFC, kg ME, MJ DMI, % of BW
Concentrate levelsa
Replacement of SBM with SRU (%)
Low
High
0
33
66
100
8.28 7.77 1.02 0.32 2.60 3.52 99.12 1.92
9.10 8.68 1.06 0.35 2.02 5.49 112.01 2.16
8.96 8.45 1.07 0.34 2.37 4.63 108.66 2.10
8.91 8.42 1.06 0.34 2.34 4.35 107.32 2.10
8.45 8.01 1.00 0.33 2.29 4.44 100.46 1.97
8.46 8.01 1.03 0.33 2.25 4.61 105.86 2.00
SEM
0.248 0.241 0.029 0.078 0.012 0.228 0.810 0.061
P-valueb Conc. SRU (Interaction)
Conc.
SRU Linear
SRU Quadratic
0.599 0.578 0.646 0.866 0.895 0.473 0.547 0.521
0.426 0.357 0.780 0.474 0.019 0.003 0.162 0.283
0.019 0.029 0.065 0.636 0.114 0.942 0.516 0.046
0.856 0.894 0.304 0.992 0.967 0.133 0.267 0.670
DM ¼dry matter, OM¼ organic matter, CP¼ crude protein, EE ¼ether extract, NDFap¼ neutral detergent fiber corrected for ashes and protein, NFC¼ nonfiber carbohydrates, ME¼ metabolizable energy, BW ¼body weight a Low¼400 g concentrate/kg of DM; High ¼800 g concentrate/kg of DM. b Effects of concentrate level (Conc.), linear effect of SRU inclusion and its interaction (Conc. SRU), and quadratic effect of SRU inclusion.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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Table 3 Effect of replacing soybean meal (SBM) with slow release urea (SRU) and concentrate levels on total and partial digestibility of dietary constituents in beef cattle.
DM OM CP EE NDFap NFC ME DM OM CP NDFap DM OM CP NDFap
Concentrate levela
Replacement of SBM with SRU (%)
Low
0
High
Total-tract digestibility (g/kg) 754.7 752.9 756.5 769.4 762.2 768.0 724.8 708.5 713.3 844.0 814.9 827.7 623.6 598.2 605.6 838.7 821.6 836.5 MJ/kg of DM 11.97 12.30 12.13 Ruminal digestibility c (g/kg) 521.8 458.9 531.6 558.6 483.1 564.0 175.2 132.2 155.1 592.8 566.0 589.6 Intestinal digestibility c (g/kg) 475.4 527.3 477.2 466.7 522.1 465.1 648.0 664.9 654.8 87.0 74.9 96.0
SEM
P-valueb Conc. SRU (Interaction)
Conc.
SRU linear
SRU quadratic
0.69 0.70 0.60 0.88 1.35 0.89
0.648 0.684 0.332 0.257 0.978 0.698
0.937 0.757 0.352 0.228 0.448 0.577
0.896 0.854 0.353 0.744 0.793 0.112
0.169 0.181 0.130 0.495 0.648 0.064
12.51
0.04
0.777
0.478
0.339
0.129
467.5 496.6 151.0 602.7
483.6 512.9 202.1 575.9
1.93 1.90 2.69 1.62
0.138 0.114 0.712 0.266
0.154 0.074 0.115 0.400
0.314 0.265 0.434 0.931
0.308 0.274 0.371 0.842
480.9 475.6 634.6 72.3
529.4 525.6 657.1 101.2
1.85 1.94 1.10 4.24
0.311 0.280 0.414 0.729
0.396 0.387 0.523 0.925
0.360 0.286 0.711 0.694
0.892 0.946 0.981 0.399
33
66
100
757.5 769.0 721.2 834.3 624.7 815.1
735.8 749.0 696.9 814.7 587.2 819.6
765.5 777.2 735.2 841.0 626.0 849.4
12.05
11.88
478.8 509.9 106.6 549.5 518.0 511.3 679.1 111.2
DM¼ dry matter, OM¼ organic matter, CP¼ crude protein, EE¼ ether extract, NDFap¼neutral detergent fiber corrected for ashes and protein, NFC¼ nonfiber carbohydrates, ME ¼metabolizable energy a Low¼ 400 g concentrate/kg of DM; High ¼ 800 g concentrate/kg of DM b Effects of concentrate level (Conc.), linear effect of SRU inclusion and its interaction (Conc. x SRU), and quadratic effect of SRU inclusion c Ruminal digestibility¼ [(nutrient intake concentration of nutrient in the abomasum)/nutrient intake]; Intestinal digestibility ¼[(concentration of nutrient in the abomasum concentration of nutrient in the feces)/concentration of nutrient in the abomasum].
NDFap intake (Table 2). Animals fed LCD had greater intake of NDFap and lower intake of NFC.
3.2. Total and partial digestibility and dynamics of ruminal degradation No differences were observed (P 40.10) for total digestibility of nutrients with exception of the NFC, which had a quadratic effect (P¼ 0.064) for the inclusion of SRU in the diet (Table 3). No effects of concentrate level in diets were observed (P40.10) for ruminal apparent digestibility of DM, CP and NDFap (Table 3). However, greater value of ruminal apparent digestibility of OM was observed (P ¼0.074) in animals fed with LCD. The inclusion of SRU did not affect (P4 0.10) any of the ruminal and intestinal parameters evaluated (Table 3). The kp of DM and OM was affected (Po0.10) by concentrate levels with greater kp for HCD (Table 4). The replacement of SBM protein with SRU did not affect (P 40.10) kp and kd of DM, OM, CP and NDFap. 3.3. Ruminal parameters There was an interaction between levels of concentrate, inclusion of SRU in the diet, and sampling time for pH (P ¼0.038). Two equations were generated to describe ruminal pH, for LCD and HCD (Fig. 2). Ruminal pH had lower values until critical point was reached in diets without SRU for animals fed LCD. For animals fed with
HCD, the ruminal pH had higher values up to the critical point to animals fed diets without SRU. The opposite behavior was observed after the critical point and the diet with 100% of replacement of SBM with SRU presented higher pH values. Sampling time had a cubic effect on concentrations of ruminal NH3–N (P¼ 0.061). The cubic behavior showed a later peak of NH3–N, followed by a decrease and an increase according to the levels of SRU (Fig. 3). However, no decrease was observed for the critical concentration of NH3–N. Concentrations of ruminal NH3–N were higher throughout the day for animals fed with LCD and total replacement of SBM protein with SRU. Animals fed HCD had the first peak in the concentration of ruminal NH3–N 2-h after feeding for treatments without SRU, and 3-h after feeding for the other levels of inclusion of SRU. The concentration of NH3–N 10-h after feeding had an opposite pattern presenting higher values for treatments with lower levels of SRU (Fig. 3). 3.4. Nitrogen balance and microbial protein production The level of concentrate in the diets did not affect (P 40.10) the concentrations of serum urea–N (SUN), N intake, N balance, efficiency of N utilization and microbial efficiency (Table 5). The SRU increased linearly N urinary excretion and decreased linearly N intake, fecal excretion of N, N balance and efficiency of N utilization. Also, SRU did not affect (P 40.10) the concentrations of SUN and the microbial efficiency.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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Table 4 Effect of replacing soybean meal (SBM) with slow release urea (SRU) and concentrate levels on passage and digestion rate of dietary constituents in beef cattle.
DM OM CP NDFap DM OM CP NDFap
Concentrate levela
Replacement of SBM with SRU (%)
Low
0
High
Passage rate (kp) %/h 3.40 4.37 3.73 3.26 4.35 3.62 5.51 6.13 5.83 1.70 1.61 1.77 Digestion rate (kd) %/h 3.77 3.87 4.22 4.17 4.25 4.65 1.41 0.85 1.06 2.36 2.04 2.34
33
66
100
4.06 3.97 6.15 1.76
4.03 3.97 6.04 1.56
3.72 3.66 5.26 1.58
3.91 4.30 0.85 2.09
3.66 4.03 1.24 2.29
3.49 3.85 1.37 2.07
SEM
P-valueb Conc. SRU (Interaction)
Conc.
SRU linear
SRU quadratic
0.15 0.16 0.19 0.07
0.195 0.157 0.498 0.502
0.006 0.003 0.108 0.676
0.962 0.898 0.279 0.248
0.158 0.150 0.150 0.986
0.21 0.22 0.19 0.12
0.237 0.225 0.727 0.311
0.851 0.895 0.141 0.330
0.113 0.194 0.429 0.391
0.831 0.813 0.667 0.930
DM ¼dry matter, OM ¼organic matter, CP¼crude protein, NDFap¼ neutral detergent fiber corrected for ashes and protein. a Low ¼400 g concentrate/kg of DM; High¼ 800 g concentrate/kg of DM b Effects of concentrate level (Conc.), linear effect of SRU inclusion and its interaction (Conc. SRU), and quadratic effect of SRU inclusion.
Fig. 2. Values of ruminal pH as a function of time and replacement of soybean meal (SBM) with slow release urea (SRU) for low and high concentrate diets. Low¼ 400 g concentrate/kg of DM; pH(low)¼ 6.8238 þ0.001836 SRU 0.19411 TIME 0.000093 SRU TIMEþ 0.008727 TIME². High ¼ 800 g concentrate/kg of DM; pH(high) ¼ 6.8238–0.00303 SRU 0.2195 TIMEþ 0.000267 SRU TIMEþ 0.008727 TIME².
Fig. 3. Values of ruminal ammonia (NH3–N) as a function of time and replacement of soybean meal (SBM) with slow release urea (SRU) for low and high concentrate diets. Low ¼400 g concentrate/kg of DM; NH3–N (low)¼ 12.0886 þ0.2704 TIMEþ 0.01546 TIME SRU 0.06891 \TIME² 0.00116 TIME² SRUþ 0.00000429 TIME² SRU²þ 0.002798 TIME3. High¼ 800 g concentrate/kg of DM; NH3–N (high) ¼12.0886 þ 0.2704 TIMEþ 0.00196 TIME SRU 0.06891 TIME² 0.00021 TIME² SRUþ0.00000429 þ0.002798 TIME3.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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Table 5 Effect of replacing soybean meal (SBM) with slow release urea (SRU) and concentrate levels on serum ureic nitrogen (SUN), nitrogen intake, fecal excretion of nitrogen, urinary excretion of nitrogen, nitrogen balance (BNC), efficiency of N utilization (Eff. N) and microbial efficiency (Mic. eff.).
SUN, mg/dL N intake, g/day N feces, g/day N urine, g/day BNC, g/day Eff. N, g/kg Mic. eff., g/kg TDN a b
Concentrate levela
Replacement of SBM with SRU (%)
Low
High
0
33
66
100
12.83 163.85 45.11 65.36 53.38 323.4 117.56
10.24 169.48 49.09 61.93 58.46 338.5 122.81
11.35 171.89 49.10 60.14 62.66 360.4 126.37
11.30 170.24 47.74 50.01 63.49 376.3 107.50
11.57 159.37 47.88 63.89 47.60 292.5 132.31
11.90 165.16 43.68 71.55 49.94 294.6 114.54
SEM
0.53 4.65 1.53 2.09 3.06 1.27 5.05
P-valueb Conc. SRU (interaction)
Conc.
SRU linear
SRU quadratic
0.430 0.646 0.566 0.101 0.565 0.402 0.135
0.182 0.780 0.503 0.613 0.628 0.628 0.788
0.443 0.065 0.038 0.007 0.003 0.005 0.822
0.745 0.304 0.422 0.132 0.815 0.777 0.816
Low¼ 400 g concentrate/kg of DM; High ¼ 800 g concentrate/kg of DM. Effects of concentrate level (Conc.), linear effect of SRU inclusion and its interaction (Conc. SRU), and quadratic effect of SRU inclusion.
4. Discussion
4.2. Total and partial digestibility
4.1. Intake
As expected, the replacement of SBM with SRU did not change the total and partial digestibility of DM, OM, CP and NDFap (Table 3). Microbial protein can meet 100% of the metabolizable protein requirements of beef cattle (NRC, 2000). This might explain the lack of effects (P 40.10) of levels of concentrate and inclusion of SRU in diets on total and intestinal apparent digestibility of CP. The absence of differences (P ¼0.822) among treatments for microbial efficiency (Table 5) support the data of total apparent digestibility of CP observed (Table 3). Cattle fed HCD had ruminal pH values lower than the recommended values that would not impair ruminal feed degradation (Owens and Goetsch, 1988). However, this low ruminal pH value was observed only during three hours within the 24 h of evaluation (Fig. 2) and was not enough to cause differences (P40.10) among LCD and HCD for total apparent digestibility of the dietary components (Table 3). Passage rate is one of the main factors that affect the efficiency of use of feed (Hall and Huntington, 2008). Thus, as a greater kp was observed when animals were fed HCD, it can be inferred that even in diets with low levels of NDFap the ruminal fermentation time was shorter in cattle fed HCD, which might explain the results observed for apparent digestibility. In this study no effect of concentrate level was observed (P¼0.40) on ruminal digestion of NDFap (Table 3). The average pH of 6.1370.30 for LCD and 5.9070.36 for HCD probably was not enough to affect the growth of cellulolytic bacteria. In addition, the high degradability of NDFap of the ingredients that composed the diet (corn, SBM and corn silage) might have influenced the values of the ruminal digestibility of NDFap observed in this trial. The digestion of protein fraction in the rumen depends on a hydrolysis rate due to the complexity of the protein molecule (Owens and Goetsch, 1988). When the rate of protein degradation exceeds the N use in the rumen for microbial protein synthesis, the excess of ammonia is absorbed through the ruminal wall. Since the values of ruminal digestibility are related to the availability of N during ruminal fermentation, it can be inferred that SRU was similar to SBM for the total N release for ruminal bacteria as no differences were observed (P¼0.434) for
We hypothesized that replacement of SBM with SRU in diets of beef cattle would not affect the synchrony of N and energy in the ruminal environment and the protein efficiency and ruminal microbial protein production. Consequently, the replacement of SBM with SRU would maintain the digestibility of the diet, as well as the DM intake of the animal. Compared with SBM, SRU decreased the intake of DM, OM and CP in beef cattle at the finishing period. However, the microbial efficiency was not affected by the use of SRU. Similarly, no changes were observed on kd and kp of DM, OM, CP and NDFap when cattle were fed diets with SRU. Corn meal was the main source of energy used and the more abundantly used among the ingredients of the concentrate, reaching levels up to 752.4 g/kg of total DM for HCD with and total replacement of SBM with SRU (Table 1). Therefore, the high digestibility of corn meal may rapidly increase the availability of energetic metabolites. This would cause a ruminal asynchrony of the availability of energy and N, which in turn was slowly released by SRU. Additionally, animals were fed once a day, which also would lead to an intake of greater amounts of corn meal in a short period of time, contributing to the appearance of ruminal acidosis. Several mechanisms have been suggested to explain how feed intake can be affected by non-protein nitrogen. The rapid hydrolysis of urea in the rumen and the increase of NH3–N absorption through the ruminal epithelium may cause animal intoxication (Huntington et al., 2006). However, concentrations of NH3–N observed in this study (Fig. 3) were below the toxic level, and it was within the range that optimizes ruminal degradation and voluntary intake in tropical conditions (Leng, 1990). In addition, the hepatic urea cycle may trigger indirectly the oxidative metabolism in the liver, which would increase the production of ATP affecting the animal feed intake (Allen, 2000). However, further studies are required to evaluate animal performance at the finishing period in order to verify the effects of SRU on intake and its impact on body weight gain.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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8
ruminal digestibility of CP among different levels of inclusion of SRU (Table 3). It should be noted that there are several other sources of ruminal N such as diffusion through the ruminal epithelium and recycling through the urea cycle (Kennedy and Milligan, 1980; Marini et al., 2004; Wickersham et al., 2009). The lack of effect (P ¼0.711) observed for intestinal digestibility of CP showed that changes in the proportion of non-protein N of the diets did not cause challenges in protein digestion in the compartments after the abomasum (Table 3). Thus, the microbial protein likely met 100% of the metabolizable protein required by the animals. 4.3. Dynamics of ruminal degradation Greater kp of DM and OM found in this study in animals fed HCD may be due to the lower fiber content in the diet compared to LCD (Table 1). Moreover, particles originated from HCD are smaller and thus have a higher density, which allows them to spend less time in the ruminal environment (Allen, 1996). The kd and kp are totally interdependent and their values determine the final results of intake and digestibility (Poppi et al., 2000). Cattle fed LCD had greater digestibility of OM (Table 3), which also explains the lower kp of OM for these animals compared to those fed HCD (Table 4). The lack of differences for kd of NDFap among treatments (Table 4) may be due to the high digestibility of fiber from the ingredients used in the experimental diets. 4.4. Ruminal parameters Ruminal pH had lower values until a critical point was reached in diets without SRU for animals fed LCD (Fig. 2), which can be explained by the fact that SBM has higher energy content than SBM, which allowed faster ruminal fermentation. This could cause changes in volatile fatty acids (VFA) profile, which in turn causes the reduction of ruminal pH (Dijkstra, 1994). Increases in ruminal VFA concentration occur as a result of a greater production of organic acids, which exceed the utilization rate as well as absorption and/or passage rates. For animals fed HCD, in diets without SRU the ruminal pH had higher values until the times where the critical point (peak of minimum values) was reached (Fig. 2). After the critical point a different behavior was observed and higher pH values were detected when animals were fed diets containing 100% of SRU in replacement of SBM, which might be explained by the greater concentration of ruminal NH3–N for these diets 10 h after feeding (Fig. 3). The relationship among pH and VGA may be modified by NH3–N in the ruminal environment (Briggs et al., 1957) and the concentration of VGA increases with the enhancement of ruminal ammonia (Wanapat and Pimpa, 1999). Owens and Goetsch (1988) reported that the pH of ruminal fluid ranges from 5.5 to 6.5 in animals fed HCD. In addition, Furlan et al. (2006) suggested that the ideal pH for the ruminal environment ranges from 5.5 to 7.0. The pH values found in this study are within the recommended values as an optimal pH value for the ruminal environment (Fig. 2).
No decrease was observed for the critical concentration of NH3–N in the rumen (Fig. 3). Among the animals fed LCD, higher concentrations of ruminal NH3–N were observed those fed diets with total replacement of SBM by SRU. In cattle fed HCD the concentration of ruminal NH3–N was higher when the animals were fed diets with lower level of SRU (Fig. 3). Ammonia concentration in the rumen represents a balance between several factors, including the absorption into the rumen wall (Tillman and Sidhu, 1969). After being absorbed NH3–N is released into the bloodstream, where it is recycled to the digestive tract or excreted in urine (Huntington and Archibeque, 1999). Thus, the NH3–N concentration in the rumen is related to excretion of N in urine. The higher values of N in urine for diets with total replacement of SBM with SRU observed in this work (Table 5) would explain why these diets had lower concentrations of NH3–N 10-h after feeding for cattle fed HCD. Concentration of ruminal NH3–N found in this study was within the minimum (Detmann et al., 2010; Leng, 1990; Satter and Slyter, 1974; Schaefer et al., 1980) and maximum (Santos and Pedroso, 2010) suggested values that do not affect ruminal fermentation and voluntary feed intake, and was bellow the toxicity level (Fig. 3). The low values of NH3–N found in this study may be a related consequence of the high energy availability provided by the experimental diets, which allowed the ruminal NH3–N to be rapidly assimilated by the ruminal microorganism. 4.5. Nitrogen balance and production of microbial protein The urinary excretion of N was linearly increased by the inclusion of SRU in the diets (Table 5). Conversely, a linearly decrease was observed on N intake, fecal excretion of N, balance of N, and efficiency of N utilization. On the other hand, no effects of SRU were observed on levels of SUN and on the microbial efficiency (Table 5). The decrease of N intake and fecal N with the increase of SRU inclusion in diets was observed as a consequence of the lower intake of DM and CP as the SRU increased in the diets. As shown in Table 2, there was a decrease of 5.8 g for DM intake and 0.58 g for CP intake for each percent unit of SRU added in the diet, since the experimental diets were isonitrogenous. The coefficient of total apparent digestibility of CP did not change among experimental treatments (Table 3), thus supporting the results described previously. The increase of urinary N loss with the increase of SRU levels in the diets might have occurred due to the rapid hydrolysis of ruminal NH3–N resulting in escape of NH3–N from the rumen (Cherdthong and Wanapat, 2010; Highstreet et al., 2010). This may have reduced the efficiency of utilization of dietary N resulting in a greater urinary loss. Indeed, the N retention was lower as the level of inclusion of SRU in the diets increased (Table 5). Since the CP sources are the most expensive components of the diet (Fernandes et al., 2007), the inefficient use of nitrogen by animals fed with SRU would increase production costs. The effects of N balance on animal performance require further investigations to ensure the level of inclusion of SRU as a replacement of SBM in order to improve tissue deposition and animal performance.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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The maximum microbial growth efficiency can be reached by maximizing the synthesis of microbial N per unit of carbohydrates fermented in the ruminal environment (Pina et al., 2010). Therefore, it can be inferred that all experimental treatments used in this trial has provided an adequate ruminal environment for microbial efficiency with values of 120.18 g/kg of TDN (Table 5). This value is lower than NRC (2001) recommendations (130 g/Kg of TDN) and close to 120 g/kg of TDN, suggested by BRCorte (Valadares Filho et al., 2010). 5. Conclusions SRU decreases intake of DM, OM and CP of beef cattle at the finishing period. However, the use of SRU as a substitute of SBM does not change the ruminal microbial efficiency, digestibility, kd and kp of diets for beef cattle at the finishing period. Replacement of SBM by SRU allows better recovery of ruminal pH after reaching low values due to the use of HCD. Compared to SBM, SRU provides higher concentrations of NH3–N throughout the entire day for cattle fed LCD. Conflict of interest statement The authors have declared that no conflict of interest exists.
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Soybean meal replaced by slow release urea in finishing diets for beef cattle P.D.B. Benedeti a,n, P.V.R. Paulino b, M.I. Marcondes a, S.C. Valadares Filho a, T.S. Martins a, E.F. Lisboa a, L.H.P. Silva a, C.R.V. Teixeira a, M.S. Duarte a a b
Department of Animal Science, Universidade Federal de Viçosa, Viçosa 36571-000, Minas Gerais, Brazil Cargill Animal Nutrition/Nutron
a r t i c l e i n f o
abstract
Article history: Received 20 August 2013 Received in revised form 12 March 2014 Accepted 24 April 2014
Eight crossbred steers (average body weight of 418 kg) fitted with ruminal and abomasal cannula were used to evaluate the effects of replacing soybean meal (SBM) with slowrelease urea (SRU) in beef cattle diets containing two concentrate levels. The experimental design included two 4 4 Latin squares, which were run simultaneously. Each Latin square received one level of concentrate [400 or 800 g/kg on a dry matter (DM) basis]. Within each Latin square, the four replacement levels of soybean meal protein with slowrelease urea were applied to the animals (0%, 33%, 66% and 100% of substitution on N basis). The DM intake as well as organic matter (OM) intake and crude protein (CP) intake decreased linearly (P o 0.05) as SBM was replaced with SRU. Ruminal digestibility coefficient of OM tended to be greater (P ¼ 0.074) for the 40 % concentrate diet. DM and OM passage rate (kp) were greater (P o 0.05) on the 80% concentrate diet. A cubic effect (P o 0.10) of SBM replacement with SRU on ruminal ammonia (NH3–N) concentration in relation to time was detected. A quadratic effect on pH was observed (P o0.10) when replacing SBM with SRU. Nitrogen intake, nitrogen excreted in the feces, nitrogen balance and efficiency of nitrogen use decreased linearly (P o0.10) as SRU increased in the diet, whereas the total nitrogen excreted in urine increased linearly (P ¼ 0.007). The production of microbial nitrogen and microbial efficiency were not affected by the experimental treatments (P4 0.10). A lower intake of DM, OM, and CP was observed when cattle were fed SRU compared to SBM. However, the use of SRU did not change the digestibility and digestion rate (kd) and kp of DM, OM, CP and neutral detergent fiber corrected for ash and protein (NDFap). In summary, SRU provides higher concentrations of NH3–N throughout a day than SBM in cattle fed low concentrate diets. & 2014 Elsevier B.V. All rights reserved.
Keywords: Cannulated Feedlot Non-protein nitrogen pH Ruminal ammonia
1. Introduction High concentrate diets (HCD) have been intensively used in feedlots in Brazil as it improves intake, body weight gain and carcass weight gain in beef cattle (Keane
n
Corresponding author. Tel.: þ1 (775) 409 9653. E-mail address: [email protected] (P.D.B. Benedeti).
et al., 2006). Most of these feeding systems use high grain diets and soybean meal (SBM) as a crude protein (CP) source (Millen et al., 2009). However, due to the high costs of soybean, beef producers have been seeking for an alternative protein source that would reduce the feeding costs. As such, the use of non-protein nitrogen (NPN) in ruminant diets appears as a viable strategy to enhance ruminal microbial protein (Storm and Ørskov, 1983) and consequently reduce feeding costs.
http://dx.doi.org/10.1016/j.livsci.2014.04.027 1871-1413/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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Urea is the main NPN source used in beef cattle diets in Brazil (Millen et al., 2009). However, there are still concerns regarding its use due to rapid release of ammonia (NH3–N), which can be faster than its use by microorganisms for protein synthesis. The efficiency of protein synthesis from urea depends, among other factors, on energy availability in the rumen (Russell et al., 1992). Further, a fast release of nitrogen may cause an asynchrony of NH3–N and energy in the ruminal environment. As a consequence, excessive NH3–N can decrease animal performance and, in some cases, cause ammonia toxicity (Bartley et al., 1976; Huntington et al., 2006; Owens et al., 1980). The use of slow-release urea (SRU) may improve the synchrony of energy and NH3–N in the rumen, leading to a better efficiency of ruminal bacteria growth. Improving the NH3–N and energy synchrony in the ruminal environment using SRU would reduce use of true protein sources in beef cattle diets. Moreover, since fiber digestion may be improved as a consequence of continuous release of nitrogen in the ruminal environment (Alvarez-Almora et al., 2012), the use of SRU appears as a great strategy to enhance the efficiency of use of fibrous feed sources. Consequently, the use of SRU may reduce feeding costs without compromising animal performance. Studies have reported efficacy in reducing NH3–N release in the rumen (Highstreet et al., 2010; Huntington et al., 2006; Owens et al., 1980), ruminal fermentation (Owens et al., 1980), microbial protein synthesis (Cherdthong et al., 2011; Xin et al., 2010), pH (TaylorEdwards et al., 2009), digestibility, and intake (Cherdthong et al., 2011) when SRU replaced conventional urea. However, the consequences of replacing sources of true protein with SRU in diets with different levels of concentrate remain unclear. Therefore, this study was performed to evaluate the effects of replacing SBM with SRU in beef cattle diets containing two concentrate levels on ruminal parameters of beef cattle. 2. Materials and methods The work described was carried out in accordance with EC Directive 86/609/EEC for animal experiments. 2.1. Animals, experiment design and diets Eight Bos indicus steers, with an average body weight (BW) of 418 740 kg and 24 months of age, were used. All animals were cannulated in the rumen and abomasum (Kehls). The animals were kept in a tie stall. All animals were initially weighed, treated for elimination of internal parasites, and then fed the experimental diets during 15 days for adaptation prior to the beginning of the experiment. Cattle adaptation to the experimental diets was performed gradually from 1% of BW followed by an increase of 0.2% of BW as dry matter (DM) every 3 days until the remaining unconsumed feed reached 10% of the DM offered. After the adaptation period, the animals were submitted to a total of four experimental periods of 15 days with 7 days for animal adaptation (Storry and Sutton, 1969) and 8 days for sample and data collection. The experimental design included two 4 4 Latin squares, which were run simultaneously. Each Latin square
had one of the two concentrate levels evaluated (60:40 roughage:concentrate ratio for low concentrate diet (LCD), and 20:80 roughage:concentrate ratio for HCD. Within each Latin square, the four replacement levels of soybean meal protein with SRU (Optigens 1200 controlled-release N, Alltechs, Araucária-PR, Brazil) were applied to the animals (0%, 33%, 66% and 100% of substitution on an N basis) totaling eight treatments with four replicates each. Experimental diets were composed of corn silage, corn meal, SBM and mixture mineral. Diets were formulated to be isonitrogenous containing 120 g/kg CP on a DM basis, in order to meet the nutritional requirements of beef steers with 400 to 500 kg of BW (Valadares Filho et al., 2010). Ingredient proportion and chemical composition of the experimental diets are presented in Table 1. 2.2. Experimental procedures and sample collections Animals were fed once daily at 0700 h, allowing for up to 10% of orts. Dry matter intake was determined from day 8 to day 15. A total feces collection was performed during three consecutive days (days 8, 9 and 10 of each experimental period) to estimate digestibility of dietary constituents, as suggested by Barbosa et al. (2006), Ferreira et al. (2009), Mezzomo et al. (2011) and Paixão et al. (2007). The variation of DM intake during the fecal collection period is presented in Fig. 1. From day 8 to 11 of each experimental period, abomasal digesta was sampled at intervals of 15 h as follows: day 8, sampling at 0800 h and 2300 h; day 9, sampling at 1400 h; day 10, sampling at 0500 h and 2000 h; and day 11, sampling at 1100 h as described by Allen and Linton (2007). Samples were frozen at 80 1C, freeze-dried for 72 h, and then ground through a 1-mm screen in a Wiley Mill. At the end of the process, a composite sample was prepared for each animal in each sampling period. Dry matter flux was determined by adding an external marker (Cr2O3) in the rumen and their concentration was measured in abomasal digesta. A daily dose (15 g) of the marker was added through the ruminal cannula at 1200 h from day 3 to 10 and the chromic oxide (Cr2O3) concentration was determined as described by Savastano (1993). Rumen evacuations for determining rumen pool size and digesta kinetics were carried out on day 12, four hours after feeding and on day 14 immediately before feeding (Allen and Linton, 2007; Mezzomo et al., 2011). Rumen contents were collected into a plastic container and separated into primarily liquid and particulate fractions by filtering through screening. Solid and liquid fractions were placed in different containers, individually weighed and sub-sampled (500 g for solids, and 2 kg for the liquid fraction) for further analysis. From day 8 to 10 urine was collected over a 24 h period using funnel collectors attached to animals with a polyethylene flexible tube that transported the urine to containers containing 250 ml of a 20% H2SO4 solution (vol:vol) to avoid loss of nitrogenous compounds (Valadares et al., 1997). Microbial biomass was determined by purine bases quantification according to Ushida et al. (1985). Ruminal liquids were sampled on day 11 to determine pH and NH3–N. Approximately 50 mL of liquid samples were manually collected from the ruminal cannula at 0, 2, 4, 6,
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Table 1 Composition and analyzed nutrient of experimental diets. Replacement of SBM by SRU (%)
Experimental diets Low concentratea
g/kg of DM Corn silage Corn meal Soybean meal SRU Mineral mixture g/kg of DM DM (g/kg) OM CP EE NDFap NFC NPN/N
High concentrateb
0
33
66
100
0
33
66
100
600 258.0 112.0 – 30
600 288.1 74.8 7.1 30
600 318.1 37.7 14.2 30
600 348.6 – 21.4 30
200 678.0 92.0 – 30
200 708.2 56.0 5.8 30
200 738.2 20.2 11.6 30
200 752.4 – 17.6 30
505.7 937.1 119.3 37.4 336.5 443.9 260.4
499.6 931.5 120.6 37.9 335.8 448.3 384.1
493.4 925.8 121.9 38.3 335.0 452.7 505.1
487.1 920.1 123.3 38.8 334.3 457.2 625.2
732.1 951.7 118.9 39.3 208.3 585.1 208.1
727.1 947.3 117.5 39.9 207.8 591.1 313.0
722.0 942.9 116.2 40.4 207.3 597.1 420.3
716.8 937.7 122.2 40.6 206.6 595.7 512.0
SRU¼ slow release urea, DM¼ dry matter, OM¼ organic matter, CP¼ crude protein, EE ¼ ether extract, NDFap ¼neutral detergent fiber corrected for ashes and protein, NFC ¼non-fiber carbohydrates, NPN/N ¼ non-protein nitrogen relation to total nitrogen a Low concentrate¼ 400 g concentrate/kg of DM b High concentrate¼ 800 g concentrate/kg of DM
Fig. 1. Average of dry matter intake during the weeks of fecal collection and replacement of soybean meal (SBM) with slow release urea (SRU) for low and high concentrate diets. Low¼400 g concentrate/kg of DM; High ¼800 g concentrate/kg of DM.
8, 10, 12, 14, 16, 18, 20 and 22 h after feeding, filtered through a triple cheesecloth layer, and pH was immediately measured by a digital pH meter (Digmed Model DM21, Digicrom, São Paulo, Brazil). After pH reading, 1 mL of a 50% H2SO4 solution (vol:vol) was added for later determination of NH3–N concentration (Chaney and Marbach, 1962). On day 15 and 4 h after feeding, blood samples were collected and immediately centrifuged at 2700 g for 20 min to obtain blood serum. 2.3. Chemical analysis and calculations Feed, feces, orts, and abomasal and ruminal samples were analyzed for DM (method 934.01; AOAC, 1990), ash (method 938.08; AOAC, 1990), CP (method 984.13; AOAC, 1990), ether extract (EE; method 920.85; AOAC, 1990), and non-protein nitrogen (NPN; Licitra et al., 1996). The organic matter (OM) was calculated as the difference between DM and ash contents. For analyzing the neutral
detergent fiber corrected for ash and protein (NDFap) concentration, the samples were treated with alpha thermo-stable amylase without sodium sulfite and corrected for residual ash (Mertens, 2002) and for residual nitrogenous compounds (Licitra et al., 1996). The levels of metabolizable energy (ME) were calculated according to NRC (2001), where ME (MJ kg 1)¼4.184 [1.01 (DCP 5.6þ DEE 9.4þDNDFap 4.2þDNFC 4.2) 0.45]. Here DCP¼digestible crude protein, DEE¼digestible ether extract, DNDFap¼digestible neutral detergent fiber corrected for ash and protein, and DNFC¼ digestible nonfiber carbohydrate. The non-fiber carbohydrates (NFC) were calculated as NFC¼100[(CP CP from ureaþUrea)þ NDFapþEEþAsh] (Detmann and Valadares Filho, 2010). Passage rate (kp) and digestion rate (kd) of dietary constituents from the data of rumen evacuation were calculated according to Allen and Linton (2007). The rate of disappearance (turnover rate) of the digesta fraction from the rumen was calculated by dividing its rate of
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intake by its ruminal pool size [ki (rate of intake; h 1)¼ intake (kg h 1) / rumen pool (kg)]. The fractional passage rate of a homogeneous fraction was calculated by dividing the flux of the fraction passing from the rumen by its ruminal pool size [kp (rate of passage; h 1)¼passage flux (kg h 1)/rumen pool (kg)]. The digestion rate of the fraction could then be calculated by subtracting its passage rate from its turnover rate in the rumen [kd (rate of digestion; kg h 1)¼ki kp]. The average rumen pool size was determined for digesta fractions by evacuating total rumen contents, weighing and sub-sampling the contents, and analyzing the sub-sample. The ruminal coefficient of digestibility was determined using the average intake and the estimated amount of DM and diet components in the abomasum. Intestinal digestibility was calculated using the estimated amount of DM and diet components in the abomasum and the amount of DM in feces (Mezzomo et al., 2011). Urine samples were analyzed for allantoin and uric acid by a colorimetric method as described by Chen and Gomes (1992). The intestinal flow of nitrogenous compounds was calculated based on the amount of microbial purines absorbed, according to the equation proposed by Chen and Gomes (1992). The flux of microbial N abomasal digest was calculated using the purine-N/total-N ratio obtained in this experiment. The microbial efficiency was calculated as total microbial biomass/TDN intake (NRC, 2001; Valadares Filho et al., 2010). Calculation of N balance was conducted using N intake from each of the three collection days (day 8 to 10) when animal feed intake was measured, minus the amount of N excreted. Excreted N was estimated based on feces and urine N content during collection days.
analyzed using the following model: Y ijkl ¼ β0 þ β1 αi þ β2 λj þ β12 αi λj þβ3 λ2j þ β13 αi λ2j þP k þAl þeijkl ; where Yijkl is the observed measurement of the ith level of concentrate in the diet and of the jth level of substitution of SBM with SRU of the kth period and of the lth animal; I¼1, 2 (levels of concentrate in the diet), j¼1, 2, 3, 4 (levels of inclusion of SRU as a replacement of SBM), β0, β1, β2, β12, β3, β13 ¼regression parameters of the model; αi ¼effect of ith fixed qualitative factor (level of concentrate, two levels); λj ¼effect of jth level of fixed quantitative factor (replacement of SBM with SRU); Pk ¼effect of level k of random factor period; Al ¼effect of level of random factor animal; eijkl ¼ unexplained residual error, assuming eijk N (0, s2), with independent errors. Only pH and NH3–N data were analyzed as a repeated measurements design. Therefore, the model tested was similar to the model above, including linear, quadratic and cubic effects of time over pH and NH3–N. All one-way, two-way and three-way interactions were tested. Outliers were identified when the Studentized residue was greater than 2.5 or smaller than 2.5. All non-significant effects (P40.10) were removed from the model to determine the final equation. In this case, the model had two errors, one random error with mean 0 and variance s2 which is the variance between animals within treatment and equal to the covariance between repeated measurements within animals; and the general residual error with the mean 0 and variance s2, which is the variance between measurements within animals.
3. Results
2.4. Statistical analysis
3.1. Intake
For statistical analysis, all results were tested for normality (Davis et al., 1989) and they all followed normal distribution (P40.05). All statistical procedures were carried out using SAS 9.2 for Windows (Statistical Analysis System Institute, Inc., Cary, NC, USA) with α¼0.10. Intake, total and partial digestibility, dynamics of ruminal degradation, nitrogen balance and microbial protein production were
Concentrate levels did not affect (P ¼0.283) DMI as a function of BW, which were 19.2 72.9 and 21.673.7 g/kg for LCD and HCD, respectively (Table 2). However, a linear decrease of DM intake as a function of BW was detected (P¼0.046) due to inclusion of SRU in the diets. The inclusion of SRU also decreased linearly the DM, CP, and OM intake. Concentrate level affected (Po0.10) NFC, and
Table 2 Effect of replacing soybean meal (SBM) with slow release urea (SRU) and concentrate levels on daily intake of dietary constituents in beef cattle.
DM, kg OM, kg CP, kg EE, kg NDFap, kg NFC, kg ME, MJ DMI, % of BW
Concentrate levelsa
Replacement of SBM with SRU (%)
Low
High
0
33
66
100
8.28 7.77 1.02 0.32 2.60 3.52 99.12 1.92
9.10 8.68 1.06 0.35 2.02 5.49 112.01 2.16
8.96 8.45 1.07 0.34 2.37 4.63 108.66 2.10
8.91 8.42 1.06 0.34 2.34 4.35 107.32 2.10
8.45 8.01 1.00 0.33 2.29 4.44 100.46 1.97
8.46 8.01 1.03 0.33 2.25 4.61 105.86 2.00
SEM
0.248 0.241 0.029 0.078 0.012 0.228 0.810 0.061
P-valueb Conc. SRU (Interaction)
Conc.
SRU Linear
SRU Quadratic
0.599 0.578 0.646 0.866 0.895 0.473 0.547 0.521
0.426 0.357 0.780 0.474 0.019 0.003 0.162 0.283
0.019 0.029 0.065 0.636 0.114 0.942 0.516 0.046
0.856 0.894 0.304 0.992 0.967 0.133 0.267 0.670
DM ¼dry matter, OM¼ organic matter, CP¼ crude protein, EE ¼ether extract, NDFap¼ neutral detergent fiber corrected for ashes and protein, NFC¼ nonfiber carbohydrates, ME¼ metabolizable energy, BW ¼body weight a Low¼400 g concentrate/kg of DM; High ¼800 g concentrate/kg of DM. b Effects of concentrate level (Conc.), linear effect of SRU inclusion and its interaction (Conc. SRU), and quadratic effect of SRU inclusion.
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Table 3 Effect of replacing soybean meal (SBM) with slow release urea (SRU) and concentrate levels on total and partial digestibility of dietary constituents in beef cattle.
DM OM CP EE NDFap NFC ME DM OM CP NDFap DM OM CP NDFap
Concentrate levela
Replacement of SBM with SRU (%)
Low
0
High
Total-tract digestibility (g/kg) 754.7 752.9 756.5 769.4 762.2 768.0 724.8 708.5 713.3 844.0 814.9 827.7 623.6 598.2 605.6 838.7 821.6 836.5 MJ/kg of DM 11.97 12.30 12.13 Ruminal digestibility c (g/kg) 521.8 458.9 531.6 558.6 483.1 564.0 175.2 132.2 155.1 592.8 566.0 589.6 Intestinal digestibility c (g/kg) 475.4 527.3 477.2 466.7 522.1 465.1 648.0 664.9 654.8 87.0 74.9 96.0
SEM
P-valueb Conc. SRU (Interaction)
Conc.
SRU linear
SRU quadratic
0.69 0.70 0.60 0.88 1.35 0.89
0.648 0.684 0.332 0.257 0.978 0.698
0.937 0.757 0.352 0.228 0.448 0.577
0.896 0.854 0.353 0.744 0.793 0.112
0.169 0.181 0.130 0.495 0.648 0.064
12.51
0.04
0.777
0.478
0.339
0.129
467.5 496.6 151.0 602.7
483.6 512.9 202.1 575.9
1.93 1.90 2.69 1.62
0.138 0.114 0.712 0.266
0.154 0.074 0.115 0.400
0.314 0.265 0.434 0.931
0.308 0.274 0.371 0.842
480.9 475.6 634.6 72.3
529.4 525.6 657.1 101.2
1.85 1.94 1.10 4.24
0.311 0.280 0.414 0.729
0.396 0.387 0.523 0.925
0.360 0.286 0.711 0.694
0.892 0.946 0.981 0.399
33
66
100
757.5 769.0 721.2 834.3 624.7 815.1
735.8 749.0 696.9 814.7 587.2 819.6
765.5 777.2 735.2 841.0 626.0 849.4
12.05
11.88
478.8 509.9 106.6 549.5 518.0 511.3 679.1 111.2
DM¼ dry matter, OM¼ organic matter, CP¼ crude protein, EE¼ ether extract, NDFap¼neutral detergent fiber corrected for ashes and protein, NFC¼ nonfiber carbohydrates, ME ¼metabolizable energy a Low¼ 400 g concentrate/kg of DM; High ¼ 800 g concentrate/kg of DM b Effects of concentrate level (Conc.), linear effect of SRU inclusion and its interaction (Conc. x SRU), and quadratic effect of SRU inclusion c Ruminal digestibility¼ [(nutrient intake concentration of nutrient in the abomasum)/nutrient intake]; Intestinal digestibility ¼[(concentration of nutrient in the abomasum concentration of nutrient in the feces)/concentration of nutrient in the abomasum].
NDFap intake (Table 2). Animals fed LCD had greater intake of NDFap and lower intake of NFC.
3.2. Total and partial digestibility and dynamics of ruminal degradation No differences were observed (P 40.10) for total digestibility of nutrients with exception of the NFC, which had a quadratic effect (P¼ 0.064) for the inclusion of SRU in the diet (Table 3). No effects of concentrate level in diets were observed (P40.10) for ruminal apparent digestibility of DM, CP and NDFap (Table 3). However, greater value of ruminal apparent digestibility of OM was observed (P ¼0.074) in animals fed with LCD. The inclusion of SRU did not affect (P4 0.10) any of the ruminal and intestinal parameters evaluated (Table 3). The kp of DM and OM was affected (Po0.10) by concentrate levels with greater kp for HCD (Table 4). The replacement of SBM protein with SRU did not affect (P 40.10) kp and kd of DM, OM, CP and NDFap. 3.3. Ruminal parameters There was an interaction between levels of concentrate, inclusion of SRU in the diet, and sampling time for pH (P ¼0.038). Two equations were generated to describe ruminal pH, for LCD and HCD (Fig. 2). Ruminal pH had lower values until critical point was reached in diets without SRU for animals fed LCD. For animals fed with
HCD, the ruminal pH had higher values up to the critical point to animals fed diets without SRU. The opposite behavior was observed after the critical point and the diet with 100% of replacement of SBM with SRU presented higher pH values. Sampling time had a cubic effect on concentrations of ruminal NH3–N (P¼ 0.061). The cubic behavior showed a later peak of NH3–N, followed by a decrease and an increase according to the levels of SRU (Fig. 3). However, no decrease was observed for the critical concentration of NH3–N. Concentrations of ruminal NH3–N were higher throughout the day for animals fed with LCD and total replacement of SBM protein with SRU. Animals fed HCD had the first peak in the concentration of ruminal NH3–N 2-h after feeding for treatments without SRU, and 3-h after feeding for the other levels of inclusion of SRU. The concentration of NH3–N 10-h after feeding had an opposite pattern presenting higher values for treatments with lower levels of SRU (Fig. 3). 3.4. Nitrogen balance and microbial protein production The level of concentrate in the diets did not affect (P 40.10) the concentrations of serum urea–N (SUN), N intake, N balance, efficiency of N utilization and microbial efficiency (Table 5). The SRU increased linearly N urinary excretion and decreased linearly N intake, fecal excretion of N, N balance and efficiency of N utilization. Also, SRU did not affect (P 40.10) the concentrations of SUN and the microbial efficiency.
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Table 4 Effect of replacing soybean meal (SBM) with slow release urea (SRU) and concentrate levels on passage and digestion rate of dietary constituents in beef cattle.
DM OM CP NDFap DM OM CP NDFap
Concentrate levela
Replacement of SBM with SRU (%)
Low
0
High
Passage rate (kp) %/h 3.40 4.37 3.73 3.26 4.35 3.62 5.51 6.13 5.83 1.70 1.61 1.77 Digestion rate (kd) %/h 3.77 3.87 4.22 4.17 4.25 4.65 1.41 0.85 1.06 2.36 2.04 2.34
33
66
100
4.06 3.97 6.15 1.76
4.03 3.97 6.04 1.56
3.72 3.66 5.26 1.58
3.91 4.30 0.85 2.09
3.66 4.03 1.24 2.29
3.49 3.85 1.37 2.07
SEM
P-valueb Conc. SRU (Interaction)
Conc.
SRU linear
SRU quadratic
0.15 0.16 0.19 0.07
0.195 0.157 0.498 0.502
0.006 0.003 0.108 0.676
0.962 0.898 0.279 0.248
0.158 0.150 0.150 0.986
0.21 0.22 0.19 0.12
0.237 0.225 0.727 0.311
0.851 0.895 0.141 0.330
0.113 0.194 0.429 0.391
0.831 0.813 0.667 0.930
DM ¼dry matter, OM ¼organic matter, CP¼crude protein, NDFap¼ neutral detergent fiber corrected for ashes and protein. a Low ¼400 g concentrate/kg of DM; High¼ 800 g concentrate/kg of DM b Effects of concentrate level (Conc.), linear effect of SRU inclusion and its interaction (Conc. SRU), and quadratic effect of SRU inclusion.
Fig. 2. Values of ruminal pH as a function of time and replacement of soybean meal (SBM) with slow release urea (SRU) for low and high concentrate diets. Low¼ 400 g concentrate/kg of DM; pH(low)¼ 6.8238 þ0.001836 SRU 0.19411 TIME 0.000093 SRU TIMEþ 0.008727 TIME². High ¼ 800 g concentrate/kg of DM; pH(high) ¼ 6.8238–0.00303 SRU 0.2195 TIMEþ 0.000267 SRU TIMEþ 0.008727 TIME².
Fig. 3. Values of ruminal ammonia (NH3–N) as a function of time and replacement of soybean meal (SBM) with slow release urea (SRU) for low and high concentrate diets. Low ¼400 g concentrate/kg of DM; NH3–N (low)¼ 12.0886 þ0.2704 TIMEþ 0.01546 TIME SRU 0.06891 \TIME² 0.00116 TIME² SRUþ 0.00000429 TIME² SRU²þ 0.002798 TIME3. High¼ 800 g concentrate/kg of DM; NH3–N (high) ¼12.0886 þ 0.2704 TIMEþ 0.00196 TIME SRU 0.06891 TIME² 0.00021 TIME² SRUþ0.00000429 þ0.002798 TIME3.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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Table 5 Effect of replacing soybean meal (SBM) with slow release urea (SRU) and concentrate levels on serum ureic nitrogen (SUN), nitrogen intake, fecal excretion of nitrogen, urinary excretion of nitrogen, nitrogen balance (BNC), efficiency of N utilization (Eff. N) and microbial efficiency (Mic. eff.).
SUN, mg/dL N intake, g/day N feces, g/day N urine, g/day BNC, g/day Eff. N, g/kg Mic. eff., g/kg TDN a b
Concentrate levela
Replacement of SBM with SRU (%)
Low
High
0
33
66
100
12.83 163.85 45.11 65.36 53.38 323.4 117.56
10.24 169.48 49.09 61.93 58.46 338.5 122.81
11.35 171.89 49.10 60.14 62.66 360.4 126.37
11.30 170.24 47.74 50.01 63.49 376.3 107.50
11.57 159.37 47.88 63.89 47.60 292.5 132.31
11.90 165.16 43.68 71.55 49.94 294.6 114.54
SEM
0.53 4.65 1.53 2.09 3.06 1.27 5.05
P-valueb Conc. SRU (interaction)
Conc.
SRU linear
SRU quadratic
0.430 0.646 0.566 0.101 0.565 0.402 0.135
0.182 0.780 0.503 0.613 0.628 0.628 0.788
0.443 0.065 0.038 0.007 0.003 0.005 0.822
0.745 0.304 0.422 0.132 0.815 0.777 0.816
Low¼ 400 g concentrate/kg of DM; High ¼ 800 g concentrate/kg of DM. Effects of concentrate level (Conc.), linear effect of SRU inclusion and its interaction (Conc. SRU), and quadratic effect of SRU inclusion.
4. Discussion
4.2. Total and partial digestibility
4.1. Intake
As expected, the replacement of SBM with SRU did not change the total and partial digestibility of DM, OM, CP and NDFap (Table 3). Microbial protein can meet 100% of the metabolizable protein requirements of beef cattle (NRC, 2000). This might explain the lack of effects (P 40.10) of levels of concentrate and inclusion of SRU in diets on total and intestinal apparent digestibility of CP. The absence of differences (P ¼0.822) among treatments for microbial efficiency (Table 5) support the data of total apparent digestibility of CP observed (Table 3). Cattle fed HCD had ruminal pH values lower than the recommended values that would not impair ruminal feed degradation (Owens and Goetsch, 1988). However, this low ruminal pH value was observed only during three hours within the 24 h of evaluation (Fig. 2) and was not enough to cause differences (P40.10) among LCD and HCD for total apparent digestibility of the dietary components (Table 3). Passage rate is one of the main factors that affect the efficiency of use of feed (Hall and Huntington, 2008). Thus, as a greater kp was observed when animals were fed HCD, it can be inferred that even in diets with low levels of NDFap the ruminal fermentation time was shorter in cattle fed HCD, which might explain the results observed for apparent digestibility. In this study no effect of concentrate level was observed (P¼0.40) on ruminal digestion of NDFap (Table 3). The average pH of 6.1370.30 for LCD and 5.9070.36 for HCD probably was not enough to affect the growth of cellulolytic bacteria. In addition, the high degradability of NDFap of the ingredients that composed the diet (corn, SBM and corn silage) might have influenced the values of the ruminal digestibility of NDFap observed in this trial. The digestion of protein fraction in the rumen depends on a hydrolysis rate due to the complexity of the protein molecule (Owens and Goetsch, 1988). When the rate of protein degradation exceeds the N use in the rumen for microbial protein synthesis, the excess of ammonia is absorbed through the ruminal wall. Since the values of ruminal digestibility are related to the availability of N during ruminal fermentation, it can be inferred that SRU was similar to SBM for the total N release for ruminal bacteria as no differences were observed (P¼0.434) for
We hypothesized that replacement of SBM with SRU in diets of beef cattle would not affect the synchrony of N and energy in the ruminal environment and the protein efficiency and ruminal microbial protein production. Consequently, the replacement of SBM with SRU would maintain the digestibility of the diet, as well as the DM intake of the animal. Compared with SBM, SRU decreased the intake of DM, OM and CP in beef cattle at the finishing period. However, the microbial efficiency was not affected by the use of SRU. Similarly, no changes were observed on kd and kp of DM, OM, CP and NDFap when cattle were fed diets with SRU. Corn meal was the main source of energy used and the more abundantly used among the ingredients of the concentrate, reaching levels up to 752.4 g/kg of total DM for HCD with and total replacement of SBM with SRU (Table 1). Therefore, the high digestibility of corn meal may rapidly increase the availability of energetic metabolites. This would cause a ruminal asynchrony of the availability of energy and N, which in turn was slowly released by SRU. Additionally, animals were fed once a day, which also would lead to an intake of greater amounts of corn meal in a short period of time, contributing to the appearance of ruminal acidosis. Several mechanisms have been suggested to explain how feed intake can be affected by non-protein nitrogen. The rapid hydrolysis of urea in the rumen and the increase of NH3–N absorption through the ruminal epithelium may cause animal intoxication (Huntington et al., 2006). However, concentrations of NH3–N observed in this study (Fig. 3) were below the toxic level, and it was within the range that optimizes ruminal degradation and voluntary intake in tropical conditions (Leng, 1990). In addition, the hepatic urea cycle may trigger indirectly the oxidative metabolism in the liver, which would increase the production of ATP affecting the animal feed intake (Allen, 2000). However, further studies are required to evaluate animal performance at the finishing period in order to verify the effects of SRU on intake and its impact on body weight gain.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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ruminal digestibility of CP among different levels of inclusion of SRU (Table 3). It should be noted that there are several other sources of ruminal N such as diffusion through the ruminal epithelium and recycling through the urea cycle (Kennedy and Milligan, 1980; Marini et al., 2004; Wickersham et al., 2009). The lack of effect (P ¼0.711) observed for intestinal digestibility of CP showed that changes in the proportion of non-protein N of the diets did not cause challenges in protein digestion in the compartments after the abomasum (Table 3). Thus, the microbial protein likely met 100% of the metabolizable protein required by the animals. 4.3. Dynamics of ruminal degradation Greater kp of DM and OM found in this study in animals fed HCD may be due to the lower fiber content in the diet compared to LCD (Table 1). Moreover, particles originated from HCD are smaller and thus have a higher density, which allows them to spend less time in the ruminal environment (Allen, 1996). The kd and kp are totally interdependent and their values determine the final results of intake and digestibility (Poppi et al., 2000). Cattle fed LCD had greater digestibility of OM (Table 3), which also explains the lower kp of OM for these animals compared to those fed HCD (Table 4). The lack of differences for kd of NDFap among treatments (Table 4) may be due to the high digestibility of fiber from the ingredients used in the experimental diets. 4.4. Ruminal parameters Ruminal pH had lower values until a critical point was reached in diets without SRU for animals fed LCD (Fig. 2), which can be explained by the fact that SBM has higher energy content than SBM, which allowed faster ruminal fermentation. This could cause changes in volatile fatty acids (VFA) profile, which in turn causes the reduction of ruminal pH (Dijkstra, 1994). Increases in ruminal VFA concentration occur as a result of a greater production of organic acids, which exceed the utilization rate as well as absorption and/or passage rates. For animals fed HCD, in diets without SRU the ruminal pH had higher values until the times where the critical point (peak of minimum values) was reached (Fig. 2). After the critical point a different behavior was observed and higher pH values were detected when animals were fed diets containing 100% of SRU in replacement of SBM, which might be explained by the greater concentration of ruminal NH3–N for these diets 10 h after feeding (Fig. 3). The relationship among pH and VGA may be modified by NH3–N in the ruminal environment (Briggs et al., 1957) and the concentration of VGA increases with the enhancement of ruminal ammonia (Wanapat and Pimpa, 1999). Owens and Goetsch (1988) reported that the pH of ruminal fluid ranges from 5.5 to 6.5 in animals fed HCD. In addition, Furlan et al. (2006) suggested that the ideal pH for the ruminal environment ranges from 5.5 to 7.0. The pH values found in this study are within the recommended values as an optimal pH value for the ruminal environment (Fig. 2).
No decrease was observed for the critical concentration of NH3–N in the rumen (Fig. 3). Among the animals fed LCD, higher concentrations of ruminal NH3–N were observed those fed diets with total replacement of SBM by SRU. In cattle fed HCD the concentration of ruminal NH3–N was higher when the animals were fed diets with lower level of SRU (Fig. 3). Ammonia concentration in the rumen represents a balance between several factors, including the absorption into the rumen wall (Tillman and Sidhu, 1969). After being absorbed NH3–N is released into the bloodstream, where it is recycled to the digestive tract or excreted in urine (Huntington and Archibeque, 1999). Thus, the NH3–N concentration in the rumen is related to excretion of N in urine. The higher values of N in urine for diets with total replacement of SBM with SRU observed in this work (Table 5) would explain why these diets had lower concentrations of NH3–N 10-h after feeding for cattle fed HCD. Concentration of ruminal NH3–N found in this study was within the minimum (Detmann et al., 2010; Leng, 1990; Satter and Slyter, 1974; Schaefer et al., 1980) and maximum (Santos and Pedroso, 2010) suggested values that do not affect ruminal fermentation and voluntary feed intake, and was bellow the toxicity level (Fig. 3). The low values of NH3–N found in this study may be a related consequence of the high energy availability provided by the experimental diets, which allowed the ruminal NH3–N to be rapidly assimilated by the ruminal microorganism. 4.5. Nitrogen balance and production of microbial protein The urinary excretion of N was linearly increased by the inclusion of SRU in the diets (Table 5). Conversely, a linearly decrease was observed on N intake, fecal excretion of N, balance of N, and efficiency of N utilization. On the other hand, no effects of SRU were observed on levels of SUN and on the microbial efficiency (Table 5). The decrease of N intake and fecal N with the increase of SRU inclusion in diets was observed as a consequence of the lower intake of DM and CP as the SRU increased in the diets. As shown in Table 2, there was a decrease of 5.8 g for DM intake and 0.58 g for CP intake for each percent unit of SRU added in the diet, since the experimental diets were isonitrogenous. The coefficient of total apparent digestibility of CP did not change among experimental treatments (Table 3), thus supporting the results described previously. The increase of urinary N loss with the increase of SRU levels in the diets might have occurred due to the rapid hydrolysis of ruminal NH3–N resulting in escape of NH3–N from the rumen (Cherdthong and Wanapat, 2010; Highstreet et al., 2010). This may have reduced the efficiency of utilization of dietary N resulting in a greater urinary loss. Indeed, the N retention was lower as the level of inclusion of SRU in the diets increased (Table 5). Since the CP sources are the most expensive components of the diet (Fernandes et al., 2007), the inefficient use of nitrogen by animals fed with SRU would increase production costs. The effects of N balance on animal performance require further investigations to ensure the level of inclusion of SRU as a replacement of SBM in order to improve tissue deposition and animal performance.
Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i
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The maximum microbial growth efficiency can be reached by maximizing the synthesis of microbial N per unit of carbohydrates fermented in the ruminal environment (Pina et al., 2010). Therefore, it can be inferred that all experimental treatments used in this trial has provided an adequate ruminal environment for microbial efficiency with values of 120.18 g/kg of TDN (Table 5). This value is lower than NRC (2001) recommendations (130 g/Kg of TDN) and close to 120 g/kg of TDN, suggested by BRCorte (Valadares Filho et al., 2010). 5. Conclusions SRU decreases intake of DM, OM and CP of beef cattle at the finishing period. However, the use of SRU as a substitute of SBM does not change the ruminal microbial efficiency, digestibility, kd and kp of diets for beef cattle at the finishing period. Replacement of SBM by SRU allows better recovery of ruminal pH after reaching low values due to the use of HCD. Compared to SBM, SRU provides higher concentrations of NH3–N throughout the entire day for cattle fed LCD. Conflict of interest statement The authors have declared that no conflict of interest exists.
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Please cite this article as: Benedeti, P.D.B., et al., Soybean meal replaced by slow release urea in finishing diets for beef cattle. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.04.027i