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* Department of Animal Sciences, Washington State University, Puyallup 98371
Abbotsford Veterinary Clinic, Abbotsford V2S 5Z5, Canada
The University of British Columbia, Vancouver V6T 1Z6, Canada
Novus International, St. Louis, MO 63141
|| H. J. Baker Company, Stamford, CT 06901
# University of Pennsylvania, Kennett Square 19348
1 Corresponding author: jhharrison{at}wsu.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: amino acid • rumen-undegradable protein • methionine • lysine
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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No work to date has incorporated addition of both different types and forms of animal and vegetable protein sources and synthetic Lys and Met sources. In an experiment by Wright et al. (2005), cows fed diets formulated with heat- and lignosulfonate-treated canola meal had significantly greater milk yields than cows fed untreated canola meal. In addition, ruminal ammonia N, BUN, urinary N (as a percentage of N intake), and MUN were reduced in cows fed diets supplemented with heat- and lignosulfonate-treated canola meal (Wright et al., 2005), indicating that treating canola meal with heat and lignosulfonate is an effective method of increasing the proportion of CP digested in the lower tract. In an experiment by Xu et al. (1998), milk production was significantly greater for cows fed diets supplemented with a commercially available blend of animal protein sources than a control diet balanced with corn distillers grains in wk 1 through 24 of lactation. Energy-corrected milk was also significantly greater for the cows fed diets supplemented with a commercially available blend of animal protein sources than a control diet from wk 9 through 24 of lactation (Xu et al., 1998).
There has been a trend in recent years for most countries to order the removal of animal-based protein sources in ruminant feeds. This increases the need for additional plant protein sources, such as heat- and lignosulfonate-treated canola meal (e.g., Amipro, Agro Pacific Industries Ltd., Chilliwack, British Columbia, Canada; Wright et al., 2005), to be evaluated as potential replacements of animal protein sources that have traditionally been used as RUP supplements. The main objective of this experiment was to determine whether heat- and lignosulfonate-treated canola meal could be used as a source of RUP while maintaining milk production in high-producing lactating cows when compared with diets supplemented with a high-quality rumen-undegradable animal protein source. The secondary objective was to determine whether milk production would be further enhanced if the diets were also supplemented with synthetic Lys and HMTBA [Alimet; contains 88% D,L-2 hydroxy-4-(methylthio)-butanoic acid and is a trademark of Novus International Inc., St. Louis, MO].
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). The grain portion of the 4 rations consisted primarily of barley and corn grains. Varying amounts of Amipro (commercially available source of heat- and lignosulfonate-treated solvent-extracted canola meal, Agro Pacific Industries Ltd.), Prolak (commercially available source of blood meal, fish meal, feather meal, and meat and bone meal, H. J. Baker, Stamford, CT), canola meal, Lys·HCl, and Alimet feed supplement [contains 88% D,L-2 hydroxy-4-(methylthio)-butanoic acid and is a trademark of Novus International Inc.] were included, depending on the dietary treatment (Table 1).
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At parturition, cows were switched from a prepartum diet to a postpartum diet. Two basal rations were formulated by using the CPM model. Amipro and Prolak were the 2 RUP sources. The chemical composition (% of DM) of Amipro was DM, 87.7; CP, 39.1; RUP, 70 (% of CP); neutral detergent-insoluble CP, 16.1; ADF, 20.4; NDF, 45.2; crude fat, 3.2; ash, 6.9; lignin, 7.6; Met, 0.56; and Lys, 1.71. The chemical composition (% of DM) of Prolak was DM, 93.3; CP, 76.9; RUP, 72 (% of CP); neutral detergent-insoluble CP, 23.1; ADF, 5.0; NDF, 17.7; crude fat, 10.3; ash, 10.8; lignin, 0 AA as a percentage of RUP; Met, 1.57; Lys, 6.05; and fatty acids as a percentage of total fatty acid DM: C16, 27.6; C18, 11.3; 18:1 trans, 1.3; 18:1 cis, 30.5; 18:2, 9.1; 18:3, 0.7. The ingredients for the 2 basal diets are shown in Table 2. The 4 postpartum treatments were 1) AMI, 2) PRO, 3) AMI+, and 4) PRO+. The postpartum levels of Met and Lys were targeted to be at approximately 100% of the requirement for the AMI and PRO diets. However, there are different ways to evaluate AA requirements for lactating dairy cattle, such as a percentage of the requirement and the Rulquin ratio (Rulquin and Verite, 1993). The CPM model uses the Rulquin ratio as one of the methods to evaluate Lys and Met requirements of lactating dairy cattle. The AMI+ and PRO+ diets were formulated based on the Rulquin ratio (Rulquin and Verite, 1993).
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Postpartum intakes of Lys·HCl and Alimet were estimated to be 40 and 20 g, respectively, for both the AMI+ and PRO+ diets. Lysine·HCL provided 8 g of ruminal escape Lys (Velle et al., 1998), and Alimet provided 7 g of ruminal escape Met (Koenig et al., 1999). The postpartum levels of Lys and Met were targeted to be at approximately 104 and 118% of requirements, respectively, for the AMI+ diet, and approximately 108 and 116% of requirements, respectively, for the PRO+ diet. The Rulquin ratio was 2.36 and 6.62% of MP for Met and Lys, respectively, for the AMI+ diet, and 2.26 and 6.65% of MP for Met and Lys, respectively, for the PRO+ diet. The Lys:Met ratios for the AMI+ and PRO+ diets were 2.81:1 and 2.94:1, respectively. The AMI+ diet was formulated to improve Met, as a percentage of MP, and Lys, as a percentage of MP, by 0.31 and 0.24, respectively, for the AMI+ diet compared with the AMI diet. The PRO+ diet was formulated to improve Met, as a percentage of MP, and Lys, as a percentage of MP, by 0.31 and 0.23, respectively, for the PRO+ diet compared with the PRO diet.
Feeding and Management
Cows were group housed, but were individually fed their respective TMR once daily through a Calan head-gate system (American Calan, Northwood, NH) both pre- and postpartum. Total mixed rations were offered to provide 10% orts, and the actual weights of TMR offered and of the orts collected were recorded daily. Cows were milked twice daily, and milk yield was recorded at each milking. Body weights and BCS were taken 3 wk prior to calving, and were recorded weekly thereafter until the end of the study. Body condition score measures were taken by the same individual throughout the course of the trial to minimize observer bias.
All cows were injected with recombinant bST (rbST; Posilac, Monsanto, St. Louis, MO) beginning at 63 DIM. The Institutional Animal Care and Use Committee at Washington State University approved this study.
Sampling and Analytical Procedures
Total mixed rations, grass silage, corn silage, and grain mixes were sampled weekly and analyzed for DM by drying in a forced-air oven (55°C). The dried samples were composited by calendar month and ground through a 1-mm screen in a Wiley mill (Arthur H. Thomas, Philadelphia, PA). Monthly composites were further pooled to result in 2 TMR samples that represented the first half and second half of the entire study period per treatment (AMI, AMI+, PRO, PRO+) for all pre- and postpartum TMR. All TMR samples were analyzed for CP, neutral detergent CP, acid detergent CP [NDF and ADF fiber recovered on Whatman 7-cm 934-AH glass microfilters from the NDF and ADF procedure were pelletized (filter and residue) and followed method 990.03; AOAC, 2000], soluble protein (Krishnamoorthy et al., 1982), ADF (method 973.18, AOAC, 1990), NDF (Goering and Van Soest, 1970, with procedure modified per D. R. Mertens, USDA-ARS, US Dairy Forage Research Center, Madison, WI; 1992, personal communication; "Use of Whatman 934-AH glass microfiber filters with 1.5 µm particle retention in place of Gooch crucibles for recovery"), crude fat (method 920.39, AOAC, 1990), ash (method 942.05, AOAC, 1990), lignin (Goering and Van Soest, 1970), and Ca, P, Mg, K, and Na (method 985.01, AOAC, 1990). Orts were sampled weekly and analyzed for DM.
Milk samples were taken weekly on Monday evening and Tuesday morning and were composited for infrared analyses of milk protein, milk fat, and milk lactose by the regional DHI laboratory (Burlington, WA). Additional milk samples were obtained at 2, 6, 10, and 14 wk postpartum for 9 cows per treatment and analyzed for MUN (Crocker, 1967), milk total N (AOAC, 1990), CN N (Rowland, 1938), and milk allantoin (Chen, 1989). Milk samples obtained from the 9 cows per treatment at 2 and 6 wk postpartum were also analyzed for milk fatty acids (Sukhija and Palmquist, 1988). Blood samples were taken from the coccygeal vein before the a.m. feeding at 2 and 6 wk postpartum for 9 cows per treatment (the same 9 cows from which additional milk samples were obtained), and concentrations of individual AA in whole blood were determined (Beckman, 1975, 1980).
Statistical Analysis
Statistical analyses were performed by using PROC MIXED of SAS (SAS, 1999). A randomized block design with repeated measures was used. Data were analyzed separately for the periods from 3 wk prepartum to parturition, 1 to 4 wk postpartum, 5 to 9 wk postpartum, and 10 to 17 wk postpartum. The first 4 wk of lactation were evaluated separately because this is a time of transition from prepartum to postpartum. Weeks 5 to 9 postpartum were evaluated as the time prior to rbST administration, and wk 10 to 17 postpartum were evaluated to observe the effects of AA supplementation post-rbST administration. Results are reported as least squares means. Significance was declared at P < 0.05 and trends were discussed when P < 0.10.
The model used to evaluate prepartum DMI and BW, and postpartum DMI, milk, and milk components for this experiment was
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where µ is the overall mean; Bi is the random effect of block (i = 1 to 25); Pj is the fixed effect of protein source (j = 1 to 2); Sk is the fixed effect of supplement source (k = 1 to 2); (P x S)jk is the interaction effect of Pj and Sk; (B x P x S)ijk is the random interaction effect of Bi, Pj, and Sk used to test the whole-plot effects [protein, supplement, and interaction (error a)]; Wl is the fixed effect of week (m = –3 to 0; m = 1 to 4; m = 5 to 9; or m = 10 to 17); (P x W)jl is the interaction effect of Pj and Wl; (S x W)kl is the interaction effect of Sk and Wl; (P x S x W)jkl is the interaction effect of Pj, Sk, and Wl; and Eijklm is the residual error (error b).
The model used to analyze milk total N, CN N, MUN, milk allantoin, milk fatty acids, and blood AA for this randomized block experiment was
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where µ is the overall mean, Bi is the random effect of block (i = 1 to 9), Pj is the fixed effect of protein source (j = 1 to 2), Sk is the fixed effect of supplement source (k = 1 to 2), (P x S)jk is the interaction effect of Pj and Sk, and Eijkl is the residual error.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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and 2, respectively. By design, the proportions of corn silage, grass silage, and alfalfa hay were similar between treatments, with forage making up approximately 70 and 42% of the dietary DM for the pre- and postpartum diets, respectively. The prepartum diets contained approximately 5% lignin (% of DM) and 2.7% crude fat (% of DM). As expected, prepartum diets were similar in mineral composition, containing on average (% of DM) Ca, 1.8; P, 0.4; Mg, 0.5; K, 1.9; and Na, 0.1.
A comparison of estimated and actual chemical composition of the test diets fed during the first 17 wk of lactation is shown in Table 2. In summary, CP, ADF, and NDF concentrations differed between estimated and actual values. Actual CP concentrations were approximately 0.5 and 1.5% greater for the Amipro- and Prolak-based diets, respectively (Table 2). Actual ADF contents were approximately 1 and 2.5% higher than estimated for the Amipro- and Prolak-based diets, respectively (Table 2). Discrepancies between actual and estimated NDF concentrations were 5 and 9% for the Amipro- and Prolak-based diets, respectively (Table 2). The average lignin was 5.0% of DM and crude fat was 4.7% of DM for each of the actual treatment diets. The postpartum diets were similar in mineral composition (% of DM), with each diet containing on average Ca, 1.0; P, 0.5; Mg, 0.4; K, 1.6; and Na, 0.4.
Postpartum requirements of Met and Lys were approximately 43 and 136 g/d, respectively. Estimated total Met and Lys available from the AMI diet were 44 and 136 g/d, respectively; from the PRO diet were 42 and 138 g/d, respectively; from the AMI+ diet were 50 and 142 g/d, respectively; and from the PRO+ diet were 49 and 144 g/d, respectively.
Intake, BW, and Milk Production
Prepartum DMI did not differ among treatments, with cows consuming on average 12.2 ± 3.8 kg/d. As expected, there were no differences in DMI during the first 4 wk, 5 to 9 wk, or 10 to 17 wk of lactation due to the source of protein (plant vs. animal; Table 3). However, DMI was significantly less (P = 0.03) when supplemented with additional Lys·HCl and HMTBA during the first 4 wk postpartum (Table 3). The reduced DMI during wk 1 through 4, when cows were supplemented with Lys·HCl and HMTBA, disappeared at wk 5 through 9 postpartum and wk 10 through 17 postpartum (Table 3).
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). However, when the data was plotted, we observed that 3.5% FCM production (Figure 2) was greater for cows fed diets supplemented with Lys·HCl and HMTBA from 14 to 17 wk postpartum. Fat-corrected milk production (3.5%; kg/d) was 39.6, 41.3, 40.1, and 42.7, respectively, for the AMI, AMI+, PRO, PRO+ groups. Milk fat production was (kg/d) 1.38, 1.45, 1.42, and 1.53, respectively for the AMI, AMI+, PRO, and PRO+ groups.
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). Milk fat content was greater (P = 0.03) and milk lactose content was lower (P = 0.03) for cows supplemented with Lys·HCl and HMTBA from wk 1 to 4 of lactation (Table 3). However, by wk 5 to 9 and wk 10 to 17 of lactation, cows supplemented with additional dietary Lys·HCl and HMTBA had milk fat and milk lactose content similar to that of the unsupplemented cows (Table 3). There was a trend for milk fat content (P = 0.10) to be greater during wk 10 to 17 of lactation for cows fed animal sources of RUP compared with vegetable sources or RUP (Table 3).
Milk Total Nitrogen, CN Nitrogen, MUN, and Milk Allantoin
Milk total N was greater (P = 0.03) for cows fed the nonsupplemented (HMTBA and Lys·HCl) treatments at wk 14 (Table 4). Interestingly, supplementation of Lys·HCl and HMTBA decreased (P = 0.05) CN N in milk samples collected at wk 2, regardless of protein source provided to the cows (Table 4). A significant protein by supplement interaction (P = 0.04) was noted at wk 14 for MUN (Table 4); supplementation of synthetic AA in the AMI+ diet resulted in a decrease in MUN concentration, whereas supplementation of these AA caused an increase in MUN concentration in milk from cows fed the PRO+ diet (Table 4). There was a trend for milk allantoin concentration to be greater at
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). In milk samples collected at 2 wk postpartum, supplementation of Lys·HCl and HMTBA to the AMI+ diet increased the concentration of 18:3 (Table 5). In contrast, the same supplementation to the PRO+ diet decreased the concentration of this particular fatty acid, resulting in a protein by supplement interaction (Table 5). This interaction was no longer evident at wk 6 postpartum because the dietary treatment did not affect the concentration of 18:3 in milk (Table 5). The concentration of conjugated linoleic acid, expressed as a percentage of total fatty acids, was greater at 2 wk (P = 0.02) postpartum for cows fed diets containing Prolak (Table 5). We also noted a similar trend at 6 wk postpartum (P = 0.08; Table 5). At 6 wk postpartum, 20:4 milk fatty acid concentration (P = 0.006) was greater for cows fed diets containing an animal protein source of RUP (data not shown). Milk fatty acid 20:4 was also greater (P = 0.02) for cows fed diets supplemented with Lys·HCl and HMTBA 6 wk postpartum compared with cows fed unsupplemented diets (data not shown).
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). The whole-blood Gly concentration was greater (P = 0.05), and there was a trend for the Ala concentration to be lower (P = 0.06) for cows fed diets containing animal sources of RUP (Table 6). The essential AA Ile was greater (P = 0.05) for cows fed diets supplemented with Lys·HCl and HMTBA at 2 wk postpartum (Table 6).
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). We also noted a similar trend for Trp (P = 0.07). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Cows fed the Amipro-based diets were able to maintain DMI, milk production, and milk components when compared with cows fed the Prolak diets throughout the 17 wk of lactation when diets were equivalent in RDP, RUP, and metabolizable Met and Lys. Liu et al. (2000) investigated the effects of feeding corn distillers grains as the sole source of RUP compared with feeding a blend of plant- and animal-based RUP sources (corn distillers grains, soybean meal, and fish meal). Similar to our findings, these authors (Liu et al., 2000) reported no differences in milk production or 3.5% FCM production.
Some researchers have investigated whether the quantity and quality of CP in the diet can affect milk production. Xu et al. (1998) evaluated 2 diets of similar CP, RDP, and RUP contents, one containing a vegetable-based source of RUP (corn distillers grains) and the other containing animal sources of RUP (blood meal, meat and bone meal, feather meal, and fish meal). These authors (Xu et al., 1998) reported no difference in DMI but an increase in milk production from cows fed diets containing animal sources of RUP. Noftsger and St-Pierre (2003) altered the digestibility of the RUP fraction by using different animal sources (porcine meal, blood meal, hydrolyzed feather meal, and poultry meal) and monitored DMI and milk production. These authors (Noftsger and St-Pierre, 2003) reported that DMI and milk production were decreased in cows fed diets containing the less digestible sources of RUP. Interestingly, in our study, there was no difference in DMI and milk production between the PRO and AMI treatments. In the study by Noftsger and St-Pierre (2003), CP content of the diet could be reduced by 1% without affecting DMI and milk production if highly digestible sources of RUP and a synthetic source of Met were included in the diet. Unfortunately, we could not validate this finding because the CP content of the AMI+ diet was only 0.3% less than the CP content of the AMI diet and there was no difference in CP content between the PRO and PRO+ diets.
As mentioned above, we observed no significant differences in milk fat concentration between the cows fed the Amipro-based diets and those consuming the Prolak-based diets. This finding is similar to the results reported by Xu et al. (1998). Wright et al. (2005), who compared the effects of heat- and lignosulfonate-treated canola meal with untreated canola meal, also found no differences in milk fat concentration. Similar to the studies by Xu et al. (1998) and Wright et al. (2005), we observed no effect of RUP source on milk protein concentration, although these authors did report differences in milk protein yield due to differences in milk yield between treatments. Supplementation of Lys·HCl and HMTBA did affect animal performance in this study; however, this effect appeared to be limited to the first 4 wk of lactation, when DMI was lower and milk fat concentration was higher, and during wk 14 17 postpartum, when increases in FCM production and milk fat production were observed in cows provided supplemental AA compared with cows that received unsupplemented diets. Xu et al. (1998) also reported that milk fat composition tended to be elevated for cows fed diets supplemented with ruminally protected Lys and Met during the first 8 wk of lactation. Rode et al. (1998) and Piepenbrink et al. (2004) reported higher milk yield and FCM when feeding 50 and 25 g of HMTBA, respectively. Similar to the current study, HMTBA was fed prior to precalving and early lactation in both of these studies (Rode et al., 1998; Piepenbrink et al., 2004).
The rise in FCM and fat production during wk 14 to 17 in the AA-supplemented animals may indicate that the combination of rbST and supplementation with Lys·HCl and HMTBA resulted in a greater use of AA for milk production, despite no differences in DMI being observed between treatment groups. Although this is only speculative at the time, we feel that this observation warrants further investigation in the future.
St-Pierre and Sylvester (2005) reported that milk production was significantly greater when high-producing cows were supplemented with an isopropyl ester of HMTBA compared with HMTBA supplementation and control. Further, these authors report that the response to milk production was progressive across the week of experiment with cows introduced to the experimental rations 4 wk postpartum. However, caution is warranted when comparing our results with those of St. Pierre and Sylvester (2005) because the sources of synthetic Met were different. Moreover, the percentage of Lys in the diet, as a percentage of microbial protein (6.8%), was considerably higher than that used in our study (6.6%). As St-Pierre and Sylvester (2005) concluded, the dietary adequacy of Lys may be a limiting factor in milk production, and thus may have been a limiting factor affecting milk production in our study.
An explanation regarding the differences in 18:3 content of milk collected from cows fed an AA supplement is not readily apparent. The increase in conjugated linoleic acid content of milk at 2 wk postpartum could be attributed to the inclusion of fish meal in the Prolak supplement, as reviewed by Chilliard et al. (2001).
Although we found that milk CN N concentration decreased 2 wk postpartum for cows supplemented with Lys·HCl and HMTBA, this difference was no longer apparent at 6, 10, or 14 wk postpartum. This finding agrees with the work of Xu et al. (1998), who reported no difference in milk CN N 4 and 8 wk postpartum between cows fed diets balanced with primarily corn distillers grains as the RUP source, diets balanced with animal protein sources for the RUP, or when synthetic Lys and Met were supplemented at 27 and 8 g/d, respectively, to the corn distillers grains-based diet. Others reported no difference in milk CN N between cows fed unsupplemented diets and ruminally available AA-supplemented diets (Bateman et al., 1999; Liu et al., 2000).
The lower values for MUN in milk from cows fed the AMI+ diet as compared with the AMI, PRO, and PRO+ diets are of interest because MUN has been linked to urinary excretion of N and reproductive performance (Jonker et al., 1998). At wk 10 and 14, MUN values in milk from AMI+-fed cows decreased, whereas those fed PRO+ produced milk with higher MUN values. According to Broderick and Clayton (1997) and Jonker et al. (1998), MUN is closely related to dietary CP concentration, with high MUN values reflecting overfeeding of CP. The protein content of the AMI+ diet was 0.3% lower (18.9 vs. 19.2%) than that of the AMI diet. However, the AMI diet was 1% lower than that of the PRO diet, with no difference in MUN values in milk collected from cows fed these treatments. The lower values of MUN in milk from the AMI+-fed cows would infer improved N efficiency, but the fate of the N is not readily apparent because no differences in BW or milk production were noted. In retrospect, it would have been of interest to monitor the reproductive performance of cows fed the different dietary treatments fed in this trial.
Considerable research has attempted to elucidate the effects on productivity of shifting the rumen microbial populations in ruminants (Vazquez-Anon et al., 2001; Noftsger et al., 2005). Milk allantoin tended to be elevated in cows fed diets supplemented with Lys·HCl and HMTBA at wk 10, which is an indication that there was an increase in microbial protein production. This trend may be due to a shift in rumen microbial populations caused by HMTBA (Vazquez-Anon et al., 2001). In lactating dairy cows, the increase in microbial protein production is a beneficial response, because the increased microbial supply of protein will provide the cow with the necessary AA to facilitate milk production.
The trend in this study for whole-blood Lys concentrations to be greater for diets supplemented with Lys·HCl suggests that further research should be undertaken to determine and validate the benefits of feeding supplemental Lys·HCl. One limitation of the current study is that we were unable to measure whole-blood Met concentrations. However, elevation of the plasma Met concentration would not have been expected when feeding HMTBA, because it is absorbed and transported in blood in the form of HMTBA and converted to L-Met within the tissues (Lobley et al., 2006).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The addition of synthetic Lys·HCl and HMTBA to diets containing either animal- or vegetable-based RUP sources did reduce DMI during the first 4 wk of lactation. However, neither milk production nor the ratio of FCM to DMI was adversely affected throughout the first 17 wk of lactation. Despite previous reports citing clear benefits of feeding supplemental synthetic AA in diets fed to high-producing dairy cows, we were unable to provide additional evidence to support these findings in the current study.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication November 7, 2006. Accepted for publication July 23, 2007.
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), PRO (Prolak, H. J. Baker Company, Stamford, CT; 
), and PRO+ [Prolak plus Lys·HCl and 2-hydroxy-4(methylthio)-butanoic acid]; 
)-based diets. SEM = 13.1 kg/d; P-value for the protein x supplement x week interaction was 0.99.