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,3
* Department of Dairy Science, University of Wisconsin, and
Agricultural Research Service, USDA, US Dairy Forage Research Center, 1925 Linden Drive West, Madison 53706
3 Corresponding author: gbroderi{at}wisc.edu from most to least effective, was in the order CM > SSBM > CSM.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: nonprotein nitrogen • true protein • production • nutrient utilization
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Milk yield was similar on diets containing CM, cottonseed meal (CSM), or SSBM (Sanchez and Claypool, 1983). This suggested that, in areas where it is available, CSM may be an economic alternative to SSBM (Blackwelder et al., 1998). However, low Lys content and availability, because of the interaction of Lys with gossypol (Craig and Broderick, 1981), may restrict CSM utilization by high-yielding dairy cows (Coppock et al., 1987; Calhoun et al., 1995; Blackwelder et al., 1998). Piepenbrink and Schingoethe (1998), who evaluated ruminal degradation and AA composition and estimated intestinal digestibilities of 4 protein supplements, concluded that CM provided an AA profile that more closely matched milk protein than did the AA profiles from blood meal, corn gluten meal, or menhaden fish meal. Previous studies (Vanhatalo et al., 1999; Kim et al., 2000; Korhonen et al., 2000) showed that His was the first-limiting AA for milk production in cows fed diets based on grass silage. Canola meal is relatively higher in His (Rode and Kung, 1996) and may be a valuable alternative to SSBM in diets fed to dairy cows.
The objectives of this trial were to compare the effects of supplementing CP as urea or 1 of 3 true protein sources that differed substantially in RUP content and AA pattern on milk production, N utilization, nutrient digestibility, and ruminal metabolism in lactating dairy cows.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Diets
The chemical composition of the silages and protein supplements fed in this trial are shown in Table 1
. Diets were fed as TMR and contained fixed proportions of alfalfa silage (AS) and corn silage (CS). Variable amounts of high-moisture shelled corn (HMSC) plus 1 of 4 protein supplements (urea, SSBM, CSM, or CM) were added to obtain diets with equal CP. Concentrates plus minerals and vitamins comprised approximately 44% of dietary DM. The TMR were prepared by blending the individual feed ingredients. The diet composition is shown in Table 2. Diets were offered once daily at 10 a.m. and orts were collected daily at 9 a.m. Amounts of feed offered to the cows were adjusted daily to allow refusals equal to 5 to 10% of intake.
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Dry matter contents of representative subsamples of weekly composites of AS, CS, HMSC, and orts, and of the weekly samples of the protein supplements, were determined by drying at 60°C (forced-air oven) for 48 h. Dry matter intake was based on the 60°C DM contents of TMR, computed from the 60°C DM contents of feed ingredients, and the orts. These samples were ground to pass a 1-mm Wiley mill screen (Arthur H. Thomas, Philadelphia, PA) and analyzed for total N (Leco FP-2000 Nitrogen Analyzer; Leco Instruments, Inc., St. Joseph, MI). Dry matter and N contents from weekly composites of the silages and HMSC and from the weekly samples of the protein supplements were used to adjust as-fed TMR composition to maintain constant dietary ingredient and CP proportions over the trial. Period composites of the dietary ingredients and TMR were prepared by mixing equal samples of DM from each of the last 2 wk of each period. These samples were analyzed for total N (Leco FP-2000 Nitrogen Analyzer), absolute DM, ash, and OM (AOAC, 1980), sequentially for NDF and ADF using heat-stable 
-amylase and Na2SO3 (Hintz et al., 1995), and for neutral detergent insoluble nitrogen (NDIN) and ADIN. Silage extracts were prepared in distilled water from weekly composites according to Muck (1987), pH was determined immediately, and extracts were analyzed for ammonia and total free AA (Broderick et al., 2004) using flow-injection (Lachat Quik-Chem 8000 FIA; Lachat Instruments, Milwaukee, WI), and for NPN (Muck, 1987; VarioMax CN; Elementar Analysensystem GmbH, Hanau, Germany).
Cows were milked twice daily and milk yield was recorded daily at each milking. Milk samples from a.m. and p.m. milkings were collected on d 19 and 26 of each period and analyzed for fat, true protein, lactose, and SNF by infrared analysis (AgSource, Verona, WI) with a Foss FT600 (Foss North America, Inc., Eden Prairie, MN) using AOAC (1990) method 972.16. For determination of MUN, separate 5-mL samples of a.m. and p.m. milk were each mixed with 5 mL of 25% (wt/vol) TCA, then vortexed and held for 30 min at room temperature before being filtered through Whatman #1 filter paper. Filtrates were stored at –20°C until MUN analysis by an automated colorimetric assay (Broderick and Clayton, 1997) adapted to flow injection (Lachat QuickChem 8000 FIA). Concentrations and yields of fat, true protein, lactose, and SNF, and MUN concentration were computed as the weighted means from a.m. and p.m. milk yields on each test day. Efficiency of conversion of feed DM was computed for each cow over the last 2 wk of each period by dividing mean milk yield by mean DMI. Apparent efficiency of utilization of feed N (assuming no retention or mobilization of body N) was calculated for each cow by dividing mean milk N output (total milk protein/6.38) by mean N intake. Body weights were measured on 3 consecutive days at the beginning of the experiment and at the end of each period to compute BW change.
Samples of whole ruminal contents (about 200 mL) were taken from the ventral sac of the rumen of the 8 ruminally cannulated cows on d 21 and 22 of each period at 0 (prefeeding), 1, 2, 4, 8, 12, 18, and 24 h postfeeding and strained through 2 layers of cheesecloth, followed immediately by pH measurement. Two 10-mL samples were then preserved by addition of 0.2 mL of 50% H2SO4 and stored at –20°C until analysis. Samples were thawed at room temperature, centrifuged (15,000 x g, 15 min, 4°C), and supernatants were analyzed for ammonia and total free AA as described for silage extracts and for VFA using GLC (Brotz and Schaefer, 1987).
Spot urine samples were obtained approximately 6 h prefeeding and 6 h postfeeding on d 20 of each period by mechanical stimulation of the vulva. After collection, 15 mL of urine was pipeted into a specimen container containing 60 mL of 0.072 N H2SO4 and stored at –20°C until later analysis. After thawing at room temperature, urine samples were analyzed for creatinine, using a picric acid assay (Oser, 1965) adapted to flow-injection (Lachat Quik-Chem 8000 FIA), for total N (VarioMax CN), for allantoin using the method of Vogels and van der Grift (1970) adapted to a 96-well plate reader, for uric acid with a commercial kit (Thermo Electron no. 1830-200, Waltham, MA; Fossati et al., 1980), and for urea with the colorimetric method also used for MUN. Daily urinary volume and excretion of urea N, total N, and purine derivatives (PD; allantoin plus uric acid) were estimated from urinary creatinine concentration assuming a creatinine excretion rate of 29 mg/kg of BW (Valadares et al., 1999). Fecal grab samples also were collected at the time of urine sampling, transferred to aluminum pans, and placed in a forced-air oven at 60°C until completely dry. Fecal samples were then ground to pass a 1-mm Wiley mill screen and composites were prepared by mixing equal amounts of DM from both samples. Fecal samples were analyzed for total N, NDF, and ADF as described for feed samples. Fecal samples and feed ingredients also were analyzed for indigestible ADF (ADF remaining after 12 d of in situ incubation; Huhtanen et al., 1994). Indigestible ADF was used as the internal marker to estimate apparent nutrient digestibility and fecal output (Cochran et al., 1986).
Blood samples were taken into heparinized tubes 4 h after feeding from the coccygeal artery or vein of each cow on d 19 of each period and stored at –20°C until analyzed. After thawing at room temperature, 5 mL of blood was transferred to a 15-mL centrifuge tube, 1.25 mL of 25% TCA (wt/vol) was added, and tubes were vortexed, held for 30 min at room temperature, centrifuged (15,000 x g, 15 min, 4°C), and the supernatants stored at –20°C until analyzed for BUN with the flow-injection system used for MUN.
Statistical Analysis
Data were analyzed using PROC MIXED in SAS (SAS Institute, 1999–2000) for a replicated 4 x 4 Latin square design. The following model was fitted to all variables that did not have repeated measurements over time:
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where Yijkl is the dependent variable, µ is the overall mean, Si is the effect of square i, Pj is the effect of period j, Ck(i) is the effect of cow k (within square i), Tl is the effect of treatment l, STil is the interaction between square i and treatment l, and Eijkl is the residual error. All terms were considered fixed, except Ck(i) and Eijkl, which were considered random. The interaction term was removed from the model when P > 0.25. Significance was declared at P 
0.05 and trends at 0.05 < P 0.10. All reported values are least squares means.
For ruminal pH, ammonia, total free AA, and VFA, which had repeated measurements over time, the following model was used:
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where Yijklm is the dependent variable, µ is the overall mean, Si is the effect of square i, Pj is the effect of period j, Ck(i) is the effect of cow k (within square i), Tl is the effect of treatment l, STil is the interaction between square i and treatment l, E1ijkl is the whole plot error, Hm is the effect of hours postfeeding analyzed as repeated measurements, HTml is the interaction between hour m and treatment l, and E2ijklm is the subplot error. The spatial covariance structure SP(POW) was used for estimating covariances, and the subject of the repeated measurements was defined as cow(square x period x treatment). All terms were considered fixed, except Ck(i), E1ijkl, and E2ijklm, which were considered random. The interaction term was removed from the model when P > 0.25. Significance was declared at P 0.05 and trends at 0.05 < P 0.10. All reported values are least squares means.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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. The AS averaged about 20% higher in CP and lower in NDF than the means reported by NRC (2001) for immature AS, indicating that it was of high quality. Composition of the SSBM was typical and, although somewhat lower in NDF and ADF, the CSM and CM had CP contents that were within about 3 percentage units of means reported in NRC (2001). As expected, NDF and ADF contents of CSM and CM were much higher than in SSBM, indicating that these supplements serve as sources of both CP and fiber. Contents of NDIN and ADIN in AS were similar to an earlier report (Brito and Broderick, 2006). Although NDIN and ADIN contents of CS were, respectively, 33 and 48% lower than in that earlier trial (Brito and Broderick, 2006), values were within expected ranges. The NDIN and ADIN contents of CM were, respectively, 62 and 52% higher than in CSM, and even greater compared with SSBM, suggesting that the CM may have been subjected to substantial heating during processing. In AS, NPN typically accounts for more than half of the total CP (Luchini et al., 1997); NPN represented only 44% of the CP in AS fed in this study. Ammonia N, total free AA N, and total NPN represented a greater proportion of total N in CS than AS. These forages usually contain similar proportions of NPN (Muck, 1987); however, AS provided 30%, and CS only 15%, of total dietary CP. Thus, AS still contributed substantially more NPN to each diet than did CS.
Crude protein contents were equal and averaged 16.6% across diets (Table 2). Neutral detergent insoluble N and ADIN were highest on the CM diet because of that protein’s greater NDIN and ADIN contents compared with SSBM and CSM (Table 1). As expected, the NRC (2001) model predicted 16% more RDP and 43% less RUP in the diet containing urea than in those supplemented with SSBM, CSM, or CM. Concentrations of NDF and ADF also were similar on urea and SSBM but higher on the CSM and CM diets, reflecting the greater fiber concentrations in those protein sources (Table 1). The low fiber and high NFC contents of the diets fed in this trial (Table 2) are discussed later in relation to ruminal pH. Net energy of lactation averaged 1.56 Mcal/kg of DM and was similar across diets.
Animal Production
Dry matter intake of cows fed urea averaged 2.5 kg/ d less (P < 0.01) than that of cows fed the true protein supplements (Table 3). Wohlt et al. (1991) and Broderick et al. (1993) reported no intake differences when feeding diets supplemented with SSBM vs. urea. Song and Kennelly (1989) also found that DMI did not differ between diets supplemented with (DM basis) 10.2% CM vs. 3.3% CM plus 0.74% urea. On the other hand, feeding SSBM instead of urea resulted in 4.6 and 1.2 kg/d more DMI, respectively, in the studies of Poos et al. (1979) and Broderick et al. (2000). Conversely, Santos et al. (1998a) observed 2.6 kg/d more DMI when cows were fed urea vs. SSBM in diets containing steam-flaked sorghum, possibly because supplementing urea with this highly degradable starch might have improved synchronization of energy and RDP fermentation in the rumen. However, when DMI data from that study were analyzed separately for high- and low-yielding cows, no significant differences were observed within the high-yielding group, whereas a trend for greater DMI was found for low-yielding cows fed urea. With relatively high-yielding cows in the present trial, it is possible that release of RDP from urea was asynchronously relative to energy from HMSC, and degradation of protein from SSBM, CSM, and CM might have been more timely relative to fermentation of the starch in HMSC. Moreover, greater nutrient demand from improved milk yield on true protein may in turn have stimulated the 10% increase in DMI over the urea diet. Furthermore, reduced DMI can result from poor palatability of urea diets (Poos et al., 1979).
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), suggesting that ruminal fill of undigested fiber did not explain the intake effect. In addition, the rate of SSBM passage was not different from that of CM passage (Brito et al., 2007b). Intake of cows fed CSM was intermediate between that of cows fed SSBM and CM. Blackwelder et al. (1998) also observed no difference in DMI when feeding cows SSBM or CSM as the only protein supplement. Moreover, both DePeters and Bath (1986) and Maesoomi et al. (2006) found only numerical differences in DMI between diets supplemented with CSM or CM.
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). Much of this response was driven by increased feed intake, and the 9 to 10% increase in feed efficiency was probably due in part to dilution of maintenance nutrient requirements. However, milk, FCM, and fat yields increased by 22 to 23%, whereas DMI increased by only 11% when true protein replaced urea. Poos et al. (1979) and Broderick et al. (2000) reported increases in milk yield of, respectively, 21 and 6% when SSBM replaced urea. Conversely, Broderick et al. (1993) observed no significant differences in milk production when SSBM replaced urea in the diet. Santos et al. (1998a) also found no effect on milk yield when all cows were included in their analysis; however, high-yielding cows fed SSBM produced 5.2 kg/d more milk than those fed urea, whereas the opposite was observed for the low-yielding group. The effect of feeding true protein on milk protein secretion was even more dramatic: Mean protein yield was increased 310 g/d, a 33% improvement. This large response can be explained by the significantly greater flows of microbial NAN and RUP (Brito et al., 2007b) with feeding SSBM, CSM, or CM vs. urea. Elevated MUN and BUN with the feeding of urea reflected the lower N efficiency on that diet.
Yields of milk and FCM averaged 41 and 38 kg/d and were not different when cows were supplemented with true protein (Table 3). Sanchez and Claypool (1983) found no significant differences in milk production when cows were fed these same protein sources, although milk yields were 1.0 and 3.2 kg/d greater, respectively, when CSM and CM replaced SSBM in their trial. Others (Mustafa et al., 1997; Blackwelder et al., 1998; Maesoomi et al., 2006) also observed no effects on milk yield when comparing SSBM vs. high- and normal-fiber CM, SSBM vs. CSM, and CSM vs. CM. Conversely, Van Horn et al. (1979) found that cows yielded significantly less milk when fed CSM rather than SSBM at 13.5% dietary CP. It is possible that, at low dietary CP, ruminal microbes benefited more from the greater RDP supply of SSBM compared with CSM, which may have led to the increased microbial protein supply and improved milk yield. Overall, most studies have shown relatively little effect of these 3 different true protein sources on milk yield.
Milk fat and lactose contents were not influenced by diet, although milk fat averaged about 0.5 percentage units below the herd mean (J. A. Davidson, US Dairy Forage Research Center, personal communication). The lower than normal milk fat content may have been related to the high concentrations of dietary NFC (Table 2) or ruminal formation, from unsaturated fatty acids in the diet, of trans-10, cis-12 conjugated linoleic acid that inhibited milk fat synthesis (Bauman et al., 2006). However, dietary fatty acid analyses were not conducted in this trial, so whether excessive intake of unsaturated fatty acids accounted for the low milk fat content could not be determined. The protein content of milk secreted by cows fed CSM was the same as for cows fed urea and lower than that of cows fed SSBM and CM (Table 3). When supplementing dairy cows with protein as CM, 50% CM plus 50% CSM, or CSM, Maesoomi et al. (2006) found the lowest milk protein content on the CSM treatment. In the present study, protein yield also was reduced by 90 g/d (P < 0.01) on CSM vs. CM, whereas cows fed SSBM were intermediate. Apparent N efficiency (milk N/N intake) was 6.3% higher on SSBM than on CSM, with cows fed CM being intermediate. Chiou et al. (1997) and Broderick et al. (2000) observed greater N efficiency feeding, respectively, SSBM vs. CSM and SSBM vs. urea. Studies have shown that Met and Lys are usually the first- and second-limiting AA for yield of milk and protein (King et al., 1990; Schwab et al., 1992a,b). According to Coppock et al. (1987), CSM had substantially lower feeding value than SSBM because of its low energy and Lys contents. Less Lys was available for absorption in the duodenum when CSM replaced SSBM (Clark et al., 1987), and cows fed SSBM had higher plasma Lys concentrations than those fed CSM (Blackwelder et al., 1998). Heat-processing of cottonseed promotes gossypol reaction with Lys 
-amino groups, reducing Lys availability and absorption (Craig and Broderick, 1981; Calhoun et al., 1995). Gossypol may not have caused a large decrease in available Lys. Autoclaving CSM for 60 min increased ruminal protein escape (estimated in vitro) from 21 to 69% (Broderick and Craig, 1980) while reducing free gossypol from 14 to 7 mg/g and nutritionally available Lys from 89 to 78% (Craig and Broderick, 1981). In vivo ruminal protein escape for the CSM fed in the present study was 51% (Brito et al., 2007b), so gossypol-Lys interaction probably resulted in Lys availability of between 78 and 89%. Blauwiekel et al. (1997) concluded that gossypol had no affect on Lys utilization or milk protein yield. Tagari et al. (1995) reported that duodenal Lys, as a proportion of total AA, was similar on SSBM and CSM. Furthermore, Rogers et al. (1984) found no differences in milk yield and milk protein content and yield when comparing abomasal infusions of SSBM vs. CSM in cows producing an average of 32 kg/d of milk. However, if Lys were first-limiting, then any decrease in its availability could impair milk production.
Blood urea nitrogen was not different among the true protein sources (Table 3). No significant changes in BUN concentration were observed comparing SSBM vs. CSM (Chiou et al., 1997; Blackwelder et al., 1998) or when SSBM, CSM, and CM were fed (Sanchez and Claypool, 1983). Conversely, Mustafa et al. (1997) reported a significant increase in BUN when SSBM replaced either high- or regular-fiber CM in the diet; however, the CP level of the SSBM diet was 1.6 percentage units greater than the average of both CM diets. Blood urea nitrogen and MUN are in equilibrium (Gustafsson and Palmquist, 1993) and, as expected, concentrations were of similar magnitude across the diets (mean BUN = 13.6 mg/dL and mean MUN = 12.6 mg/dL). High ruminal ammonia concentration (Table 4) explained the effect of diet on BUN: Compared with the mean of the 3 true protein diets, ruminal ammonia was 34% higher and BUN 32% higher on the urea diet. However, MUN was lower on CSM vs. SSBM and CM (Table 3), but there were no differences in ruminal ammonia (Table 4). Dietary CP content and intake influence MUN (Broderick and Clayton, 1997; Nousiainen et al., 2004), but CP intake on CSM was intermediate between SSBM and CM (Table 3). However, N digestibility was lower on CSM than SSBM and CM (Table 4); thus, apparent N absorption paralleled MUN concentration.
Ruminal Metabolism and Apparent Total Tract Digestibility
Ruminal pH did not differ and averaged 6.54 across diets over the 24-h feeding cycle (Table 4). Furthermore, mean ruminal pH was above 6.0 at all sampling times on all diets (Figure 1), suggesting that the ruminal environment was stable despite the relatively low NDF and high NFC contents of these diets (Table 2). The nonstarch NFC fed in this trial probably was largely fermentation acids because these diets were composed mainly of silage and high-moisture corn (Schwab et al., 2003). Thus, effective NFC levels were lower than the values computed in Table 2. As expected, ruminal ammonia concentration was greatest (P < 0.01) on the urea diet but was similar on the true protein sources. Blackwelder et al. (1998) observed higher ruminal ammonia in cows fed SSBM than in cows fed CSM, which the authors speculated might be a consequence of more rapid degradation of SSBM protein. Although ruminal ammonia concentrations did not differ on the 3 true proteins, the estimated extent of ruminal protein degradation was greater for SSBM (71%) and CM (66%) than for CSM (49%; Brito et al., 2007b), suggesting that degraded protein was rapidly utilized for microbial protein formation in our study. Ruminal total free AA concentration on SSBM was 17 and 27% higher than, respectively, CSM and CM, whereas the urea diet was intermediate. The greater degradation of SSBM protein probably also influenced this trait. Broderick et al. (2000) reported similar concentrations of ruminal free AA when lactating cows were fed urea or SSBM. Ruminal isobutyrate, which is formed partly from catabolism of branched-chain AA (Hoover, 1986), also was higher on SSBM and CM, the 2 proteins with the greatest estimated ruminal degradations (Brito et al., 2007b). No other significant effects of diet on ruminal VFA were detected.
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), which may have been a consequence of slower passage rates associated with the 10% lower DMI on urea. Greater digestibilities of NDF and hemicellulose on the CM diet probably reflected higher digestibility of CM fiber vs. fiber from other dietary components. Canola meal NDF contributed nearly 4% of dietary DM, whereas SSBM and CSM accounted for, respectively, about 1 and 3% of dietary DM (Tables 1 and 2). Lower N digestibility on CSM than SSBM and CM (Table 4) suggested that utilization of the greater RUP in CSM (Brito et al., 2007b) was impaired. No significant differences were observed for apparent ADF digestibility, which averaged 35% across diets.
Nutrient Utilization and Excretion
Total urinary N excretion was greater (P < 0.01) on urea than on diets supplemented with true protein (Table 5), which was expected because of its greater ruminal ammonia (Table 4) and lower N efficiency (Table 5). Expressed as a proportion of intake, total urinary N excretion also was greatest on the urea diet, intermediate on SSBM and CM, and lowest on CSM. After urea hydrolysis, ammonia not captured by the ruminal microbes is absorbed from the digestive tract, reconverted to urea in the liver, and either recycled back to the digestive tract or excreted in the urine. Total urinary N excretion was 25 and 26% greater, respectively, in early and midlactation cows when urea replaced SSBM (Wohlt and Clark, 1978). However, Poos et al. (1979) and Robinson et al. (1998) reported that total urinary N excretion was similar on diets containing urea or SSBM. Among the true protein sources, cows fed CSM had the lowest urinary excretion of total N. This effect may have been due to higher RUP in CSM: RUP as a proportion of DM (Table 2) and RUP flow (Brito et al., 2007b) both were greatest on the CSM diet. In previous trials, higher RUP flow (Brito et al., 2006, 2007a,Brito et al., b) was related to reduced urinary N excretion (Brito and Broderick, 2006; Broderick et al., 2007). According to Castillo et al. (2001), increasing dietary RUP diverted N excretion from urine to feces. This also occurred in the present trial because cows fed CSM excreted more N in feces than cows fed SSBM (Table 5). Because urinary N is less desirable due to its greater tendency to leach and to volatilize as ammonia, shifting N excretion from urine to feces is desirable.
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); more of the total urinary N was excreted as urea N on the urea diet (90%) than on the diets supplemented with true protein (75%). Broderick et al. (2007) observed that cows consuming lower NPN red clover silage diets excreted less urinary urea N than those consuming higher NPN AS diets, and Brito and Broderick (2006) found a linear reduction in excretion of urinary urea N when CS plus SSBM replaced AS plus HMSC. Reynal and Broderick (2005) also reported a linear decline in urinary urea excretion when lignosulfonate-treated soybean meal replaced dietary urea. Among the true protein supplements, cows fed CSM excreted, respectively, 14 and 12% less (P < 0.01) urea N in urine than those fed SSBM and CM.
Because lower urinary N excretion occurred on the same diet (CSM) that had reduced fat and protein yields vs. the other true proteins, it was not possible to minimize N excretion in this trial without compromising production of milk components. Previous trials (Brito and Broderick, 2006; Broderick et al., 2007) also showed that feeding greater RUP, as a proportion of dietary DM, was consistently associated with both decreased N excretion and with reduced production. Thus, it is important to determine the dietary RDP and RUP contents that would maximize N utilization. At dietary RDP and RUP levels of (DM basis) 12.2 and 4.3% on SSBM and 12.3 and 4.4% on CM, production and N efficiency were greater, but total urinary N excretion also was increased, compared with the CSM diet, which contained 11.2% RDP and 5.4% RUP (Table 2). Thus, it can be hypothesized that production would be maximized and N excretion minimized when diets contained 11.2% < RDP < 12.2 and 4.3% < RUP < 5.4%.
No significant differences were observed for urinary excretion of allantoin and uric acid, which averaged 377 and 32 mmol/d across diets (Table 5). However, cows fed CM tended (P = 0.09) to excrete less total PD in urine than cows fed the other protein supplements. Broderick et al. (2000) also reported no significant differences in urinary PD excretion when comparing urea vs. SSBM as protein supplements to lactating dairy cows. Urinary allantoin averaged 92% of the total PD among diets and was similar to previous reports (Vagnoni et al., 1997; Valadares et al., 1999; Broderick et al., 2000). Microbial protein synthesis estimated from PD did not differ and averaged 231 g/d across diets (Table 5).
Fecal N excretion was lower (P < 0.01) when fed urea compared with SSBM, which in turn was lower than that on CSM, whereas CM was intermediate (Table 5). However, fecal N excretion did not change significantly when SSBM replaced urea in the studies of Wohlt and Clark (1978), Poos et al. (1979), and Robinson et al. (1998). Total N excretion (feces plus urine) did not differ across diets and averaged 343 g/d (Table 5). However, about 60% of ingested N was excreted in the feces plus urine on the urea diet, which was greater (P < 0.01) than that on the diets supplemented with SSBM, CSM, or CM (mean 53%). Nitrogen efficiency, expressed as kilograms of milk yield/kilogram of total N excreted, also was lower (P < 0.01) on the urea diet (Table 5). In previous studies, total N excretion, as a proportion of N intake, averaged 74% (Broderick et al., 2007), 66% (Brito and Broderick, 2006), and 70% (Castillo et al., 2001) vs. 54% in the present trial. This discrepancy might be related to an overestimation of apparent CP digestibility. On average, apparent total tract digestibility of CP was 32 and 8% higher in the current trial than was found, respectively, by Brito and Broderick (2006) and Broderick et al. (2007). Moreover, total urinary N excretion also was 17 and 27% lower in this trial than in the previous studies.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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2 Current address: Agriculture and Agri-Food Canada, PO Box 90, 2000 Route 108 East, Lennoxville, QC, Canada.
Received for publication August 27, 2006. Accepted for publication December 5, 2006.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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A. F. Brito, G. A. Broderick, and S. M. Reynal Effects of Different Protein Supplements on Omasal Nutrient Flow and Microbial Protein Synthesis in Lactating Dairy Cows J Dairy Sci, April 1, 2007; 90(4): 1828 - 1841. [Abstract] [Full Text] [PDF]
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