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Department of Animal Sciences The Ohio State University, Columbus 43210
Corresponding author:
R. St-Pierre; e-mail:
st-pierre.8{at}osu.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSStatistical AnalysisRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: rumen undegradable protein • methionine • protein digestibility
Abbreviation key: HMB = 2-hydroxy-4-(methylthio)-butanoic acid, HiCP = 18.3% CP concentration, HiDRUP = supplemental RUP selected for high postruminal digestibility, LoCP = 17.0% CP concentration, LoDRUP = supplemental RUP unselected for digestibility, MP = metabolizable protein, MUN = milk urea nitrogen, PUN = plasma urea nitrogen, WOT = week of trial
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSStatistical AnalysisRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Efficiency of utilization could be improved by increasing postruminal digestibility and/or providing a pattern of absorbed AA that more closely matches the AA requirements for milk synthesis. O’Mara et al. (1997) found that fish meal had a high rate of disappearance of total AA in the intestine, 97%, whereas corn gluten feed was much less digestible at 75.6%. Wright et al. (1998) provided a ruminally protected supplement that contained methionine, lysine, phenylalanine, histidine, and threonine in a ratio similar to that of bovine caseins. Milk production and protein production responded positively in a linear fashion to increasing levels of RUP when supplemented with AA.
Methionine and lysine have been reported to be limiting AA for milk yield and protein production (Schwab et al., 1992; Maas et al., 1998; Bach et al., 2000). Due to the high cost of production of synthetic rumen-protected lysine, blood meal, or some other feed high in rumen undegradable lysine is generally used as a source of metabolizable lysine, whereas synthetic rumen-protected methionine can be made relatively cheaply. Supplementation of methionine or methionine and lysine postruminally has had positive effects on milk production and milk protein concentration (Armentano et al., 1997; Dinn et al., 1998; Varvikko et al., 1999).
2-Hydroxy-4-(methylthio)-butanoic acid (HMB), is a common source of Met (Schwab, 1998) that varies in estimated rumen degradability from 99% (Jones et al., 1988) to as low as 21 to 50% (Koenig et al., 1999; Vazquez-Anon, 2001). The most consistent response to feeding HMB has been an increase in milk fat percentage (Patton et al., 1970; Holter et al., 1972; Huber et al., 1984), with some increases in milk yield (Patton et al., 1970; Polan et al., 1970). Most research done before 1988 used the Ca salt of HMB, which was not completely water soluble. The liquid form of HMB currently in use is completely water soluble.
We hypothesized that milk production and composition could be maintained and dietary CP decreased to improve efficiency of N utilization through selection of high digestibility RUP and supplementation of Met. Experiment 1 was designed to assess milk production responses to changes in postruminal RUP digestibility, metabolizable protein (MP) supply, and methionine supplementation. A digestibility trial (experiment 2) was designed to measure changes in N utilization and efficiency using the same dietary treatments as in experiment 1. To test our hypothesis, we 1) maintained diet RUP, while increasing MP concentration through higher intestinal digestibility of RUP, 2) lowered diet RUP, while maintaining MP concentration through higher intestinal digestibility but without consideration to the AA balance of the undegraded feed N, and 3) lowered diet RUP, while maintaining MP concentration combined with Met supplementation in both rumen-protected form (Smartamine M) and rumen active form (Rhodimet AT-88) to bring calculated Lys to Met ratio of MP near 3:1.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSStatistical AnalysisRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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) were fed as TMR and contained on average 32.5% NDF, 19.7% ADF, and 1.60 Mcal/kg NEL with RDP at 10.5% of the DM (NRC, 2001). Diets were made up of 50% forage, with 37.5% corn silage (32.0% DM, 8.3% CP, 46.1% NDF) and 12.5% alfalfa haylage (46.0% DM, 21.6% CP, 38.0% NDF) on a DM basis. Rumen undegraded protein was either at a high concentration, 41.5%, or a low concentration, 37.5% of total CP. Supplemental undegraded protein was provided by animal protein sources that were screened to be either highly digestible in the intestine or were an unselected source (i.e., porcine meat meal) of a lower estimated digestibility in the intestine. Intestinal digestibility of supplemental RUP sources were estimated before trial initiation using the method of Calsamiglia and Stern (1995). All treatments were similar in concentration of RDP. Treatments were 1) 18.3% CP with 42% RUP of low intestinal digestibility (HiCP-LoDRUP); 2) 18.3% CP with 41.3% RUP of high digestibility (HiCP-HiDRUP); 3) 16.9% CP with 37.7% RUP of high digestibility (LoCP-HiDRUP), and 4) 17.0% CP with 37.6% RUP of high digestibility plus supplemental Met from a combination of Smartamine M and HMB (LoCP-HiDRUP + Met). Treatment diets contained supplemental RUP as either porcine meat meal (low intestinal digestibility) or a combination of high postruminal digestibility hydrolyzed feather meal, poultry meal, and blood meal. The porcine meat meal used in our HiCP-LoDRUP diet was an unselected source of RUP and was similar in estimated postruminal digestibility to values reported by others (Calsamiglia and Stern, 1995). The high digestibility RUP sources were selected from the top 10 sources among 79 tested using the Minnesota three-step enzymatic analysis of RUP (Calsamiglia and Stern, 1995). The poultry meal and blood meal used had greater than 90% estimated postruminal RUP digestibility, while the feather meal had greater than 85% postruminal digestibility. Nutrient compositions of the supplemental RUP sources are updated in Table 2. Methionine was supplied in two forms: Smartamine M, a 90% rumen-escape source of methionine and Rhodimet 88, a source of HMB. In our calculations, we assumed a 5% ruminal escape rate for HMB (Charles G. Schwab, personal communication). Concentrate ingredients, including RUP supplements and HMB, were pelleted.
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). The control ration contained Lys and Met in a ratio of 3.4:1. Lowering the RUP in treatment three resulted in the same concentration of MP as the control ration but with a Lys to Met ratio of 3.7:1. The addition of supplemental Met in treatment four brought this ratio to 3.3:1, which is closer to the optimum ratio derived by the NRC (2001). Comparisons of estimates by the CPM (1998) model with the NRC (2001) model are shown in Table 3.
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Experiment 2: Digestibility trial.
Six cows from each treatment were held for an extra 5 d in metabolism stalls for a digestibility study. Cows were selected from six blocks, balanced for parity within each block (three multiparous and three primiparous blocks) and within 2 wk of having the same DIM. Total collection of feces and urine, along with milk weights and composition, were used to determine N excretion. Cows remained on the same treatment as the one they were assigned to during the production trial. The feeding protocol was similar to that of the production trial, except that orts were collected separately and analyzed to determine exact intakes of DM, CP, and NDF.
Sample Collection
Experiment 1: Production trial.
Corn silage, alfalfa haylage, whole linted cottonseed, premixes, and pelleted feeds were sampled monthly for the 8 mo of trial and kept frozen at -20°C until analyzed. Cows were milked twice daily, with milk weights recorded at each milking. Milk samples were collected weekly at four consecutive milkings and preserved with 2-bromo-2-nitropane-1, 3-diol and refrigerated until analyzed after the fourth milking. Blood samples were collected via the coccygeal vein and arteries at 0, 5, and 10 wk of trial for plasma urea N analysis. Blood samples were collected approximately 2 h postfeeding and placed on ice for transport to the laboratory, where they were immediately centrifuged and the plasma was removed. Blood plasma was stored at -20°C until analyzed. Cows were weighed, and body condition scored on a scale of 1 to 5 once a week throughout the trial.
Experiment 2: Digestibility trial.
Total mixed rations and orts were sampled daily and composited by cow. Milk weights were recorded daily, and samples were taken at each milking on d 2 through 5 of the total collection. Feces were sampled daily, and weights recorded. Daily samples were kept refrigerated for the duration of the collection period. At the end of each collection period the samples were composited by cow and kept frozen at -20°C until analyzed.
Urine was collected using an external harness as described by Kauffman and St-Pierre (2001). Urine weight was recorded daily, and a 500-ml sample was taken and kept refrigerated. Aliquots were composited by volume, frozen, and stored at -20°C until analyzed.
Sample Analysis
Ingredient and TMR samples were analyzed for CP (AOAC, 1990), NDF, ADF, and lignin (Van Soest et al., 1991). Analyses of HMB and supplemental DL-Met (Smartamine M) were done on a monthly basis by Aventis Animal Nutrition (Coventry, France). Wet samples were dried for 48 h at 55°C and ground using a Wiley Mill (Arthur H. Thomas, Philadelphia, PA) with a 2-mm screen. Concentrate samples were ground immediately. Milk samples were analyzed for total protein, fat, SCC, and milk urea nitrogen (MUN; DHI Cooperative, Inc., Powell, OH). Milk urea N was determined by a diacetyl monoxime assay on a Skalar SAN Plus segmented flow analyzer (Skalar, Inc., Norcross, GA).
Plasma samples were assayed for plasma urea N (PUN) using Sigma kit # 535 (Sigma Diagnostics, St. Louis, MO). Both fecal and urine samples from the digestion trial were analyzed for N using the Kjeldahl method (AOAC, 1990). Fecal samples were analyzed for N on a wet basis to minimize possible losses of NH3. A sample was also dried at 55°C and analyzed for NDF (Van Soest et al., 1991).
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Statistical Analysis |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSStatistical AnalysisRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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 | ([1]) |
where
jis the mean covariate measurement for the jth parity,
Errors within cows across weeks, which are repeated measures due to multiple sampling of milk, intake, and blood, were modeled using a first-order autoregressive covariance structure. This structure consistently gave the lowest Bayesian information criteria of four covariance structures tested: unstructured, compound symmetry, first-order autoregressive, and simple (Littell et al., 1996). Gross feed efficiency was calculated as weight of milk per unit of DMI and analyzed according to model [1]. To test whether changes in gross feed efficiency were the results of treatment effects on body energy mobilization and replenishment, BW and BCS (Wildman et al., 1982) were also analyzed according to model [1]. Marginal feed efficiency was modeled by fitting [1] with milk production as the dependent variable, with the following term added to the linear model:
 | ([2]) |
where
is the overall DMI mean.
The partial differential of the milk production function ([1] augmented with [2]) with respect to DMI provides an estimate of marginal feed efficiency for each treatment. Significance was determined at P < 0.05 for the production trial.
Experiment 2: Digestibility trial.
Total collection data were analyzed as a randomized complete block design with the effect of blocks modeled as a random effect using the MIXED procedure of SAS. Significance was determined at P < 0.10 for the digestibility trial. Means were separated using Fisher’s protected LSD.
True N total tract digestibilities (Figure 1) were estimated using a linear regression of N absorbed on N intake (Van Soest, 1994) according to the following model:
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 | ([3]) |
where
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSStatistical AnalysisRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Intake and milk production.
Results for DMI and milk production measurements are reported in Table 4. The three groups receiving HiDRUP had greater DMI than the LoDRUP control (P = 0.04). This is in contrast to a review by Santos et al. (1998) who found that feeding a high-quality RUP (fish meal) with a Lys:Met ratio of approximately 3:1 had no effects on DMI over a low-quality RUP (corn gluten meal, low Lys) in eight out of nine studies reviewed. However, fish meal can become unpalatable at higher levels and may prevent an increase in intake. Milk yield was highest for the HiCP-HiDRUP and the LoCP-HiDRUP + Met diets, indicating that AA other than Met were in greater excess for the higher CP diet.
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Fat production and percentage had a significant treatment x parity interaction. Fat production in primiparous cows was higher for the HiDRUP treatments, with the greatest increases in the HiCP-HiDRUP and LoCP-HiDRUP + Met diets. Multiparous cows followed a similar trend, but fat production for the diets with HiDRUP was not significantly different. Fat percentage increased with the diets containing the HiDRUP supplements for first-parity cows, with the LoCP-HiDRUP + Met being numerically highest, although it was not significantly different from the LoCP-HiDRUP diet. Treatments had no effect on milk fat concentration in cows of second or greater parity, explaining the treatment x parity interaction. Abomasal infusions of Met have increased milk fat concentration (Varvikko et al., 1999). Polan et al. (1970) found an increase in milk fat production and concentration from supplemental HMB in the diet in amounts up to 94 g/d but with peak milk yield occurring at a supplementation of 25 g/d. Lundquist et al. (1983) found that supplementation of HMB increased fat yield and concentration at several concentrations of dietary protein and two forage-to-concentrate ratios. These authors also combined data from six lactation studies in which cows were fed either HMB or a control diet. The addition of 0.25 to 0.30% HMB to diet DM increased milk fat by an average of 0.35% and yield by 0.09 kg/d. The data analyzed included both primiparous and multiparous cows. The concentration fed was two to three times higher than that fed in the current trial and may not represent the effects of current feeding practices.
Gross feed efficiency (kilograms milk per kilogram DMI) was numerically highest for the HiCP-HiDRUP and LoCP-HiDRUP + Met treatments (P = 0.04), although they were not significantly different from the feed efficiency on the HiCP-LoDRUP diet. The HiCP-LoDRUP and LoCP-HiDRUP diets were not significantly different (P > 0.05). Higher feed efficiency indicates a better utilization of the nutrients ingested, either through increased digestibility, improved conversion of absorbed nutrients into milk, dilution of maintenance requirements, or mobilization of body reserves. The evolution of BCS across weeks was not affected by treatments (P = 0.84) and averaged 2.85 (SEM = 0.04) and 2.95 (SEM = 0.04) at wk 1 and 12, respectively. Likewise, treatments did not affect the evolution of BW through time, (P = 0.86) with a mean BW of 575 (SEM = 8.9) and 624 kg (SEM = 8.9) at wk 1 and 12, respectively. Thus, treatment effects on gross feed efficiency were not the results of different body energy reserve depletion and repletion across treatments. Treatments did not affect the marginal feed efficiency (P = 0.14), which averaged 0.39 kg/kg (SEM = 0.05) across the four treatments.
Table 5 reports calculated balances and flows of important nutrients using the NRC (2001) model with pretrial estimated milk production (45.5 kg/d) and DMI (23.8 kg/d) compared to calculations using actual least squares means of milk production, composition, and DMI during the course of the 12-wk trial. Cows were allowed feed ad libitum, and the increase in DMI was a response to the treatment diets. This table illustrates that the lower than predicted milk yield in the HiCP-LoDRUP group brought the balance of MP closer to zero, emphasizing that this diet was too low in MP originally to support the desired milk yield and indirectly supporting the accuracy of the NRC requirements. However, the LoCP-HiDRUP + Met predicted an MP balance that was more deficient than originally predicted due to increases in milk yield. This increase in milk yield, while deficient in MP, indicates that overall quantity of MP may not be as important as supplying the proper balance of Met and Lys.
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). Cows on the HiCP-HiDRUP diet consumed significantly more N than those on the other three treatments. The effect of treatments on milk N production interacted with parity, as explained previously. Gross N efficiency (milk N/N intake * 100) was increased in all diets containing high digestibility RUP compared to the diet with low digestibility RUP, with the greatest increase occurring with the Met-supplemented diet. Because N intake was similar for the LoCP-HiDRUP and LoCP-HiDRUP + Met diets, Met supplementation appeared to improve the efficiency of conversion of MP into milk N. St-Pierre and Thraen (1999) proposed using N excreted (i.e., N in feces and N in urine) over milk N as a measure of environmental N efficiency. Decreasing the environmental N efficiency has the potential of lowering the impact of dairy production on the environment. Environmental N ratio was lower for the three diets containing high intestinal digestibility RUP, with the greatest decrease occurring in the diet supplemented with Met. This indicates that feeding diets supplemented with a source of high digestibility RUP, especially in a diet formulated to be low in MP and balanced for Met, could decrease significantly the urinary and fecal N released into the environment. Higher CP diets had greater concentrations of MUN. Plasma urea N was greatest for the HiCP-HiDRUP diet, probably due to the higher N intake on this diet.
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Experiment 2: Metabolism Trial
Twelve primiparous and 12 multiparous cows were used in six blocks in the metabolism trial. One multiparous cow from the LoCP-HiDRUP + Met treatment was removed due to sharp decreases in milk production and DMI during the digestibility trial. Because of the small number of animals per treatment, the collection trial did not show the significance seen in the production trial. However, the trends support the conclusions drawn from the production trial.
Intake and digestibility.
Dry matter intake, DM digestibility, and NDF digestibility were not significantly different among treatments (Table 7). Cows were in wk 15 or 16 of lactation during the collection trial and had higher DMI than the mean DMI observed in the production trial during which cows averaged 7 to 8 wk in milk. Apparent digestibility of N did not differ (Table 8). The RUP sources provided approximately 10% of the N in the diet. Therefore, a 20% increase in the digestibility of the supplemental RUP source would only increase overall N digestibility by 2%. With an SEM for apparent N digestibility of 2.1, the power of the test was too low to see differences. The Lucas test (Van Soest, 1994) was performed to determine if the true digestibilities of N of the four treatment diets differed (Figure 1). Regressions were done by parity in order to determine metabolic fecal N for primiparous and multiparous cows. All four treatment diets were found to have 80% (± 0.5) true digestibility of N. Estimates of metabolic fecal N excretion (intercept of regression) were 103 g/d for primiparous and 132 g/d for multiparous cows. These equate to 5.3 and 5.0 g/kg DMI for primiparous and multiparous cows, respectively. The NRC (2001) uses a value of 4.7 g/kg DMI metabolic fecal N based on data from Swanson (1977). Metabolic fecal N is the portion of fecal N that can not be changed by nutrition.
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. Milk production and milk protein production were not significantly different, although there was a trend (P = 0.18) for milk protein percentage to increase in the two LoCP-HiDRUP diets. Milk and protein production by cows in the digestibility trial were lower than those observed in the production trial for cows on the LoCP-HiDRUP + Met treatment, apparently supporting the lower metabolizable Met requirement for the cows in the metabolism trial so that it was not limiting for any treatments. Nitrogen intake was numerically highest in the HiCP-LoDRUP and HiCP-HiDRUP diets, although the HiCP-HiDRUP N intake was not significantly different from the LoCP-HiDRUP and LoCP-HiDRUP + Met treatments. Nitrogen apparently absorbed was significantly different among treatments with the two HiCP-LoDRUP and HiCP-HiDRUP absorbing greater amounts (P = 0.04) than LoCP-HiDRUP and LoCP-HiDRUP + Met treatments. Intake of N by cows on the HiCP-LoDRUP diet was significantly larger than that of the LoCP diets, with the HiCP-HiDRUP group being intermediate in intake. The three rations with HiDRUP did not have significantly different N absorption, which reflects the lack of differences seen for these treatments in N intake data. The amount of N in the feces was not significantly affected by the treatments. Urinary N was lowered by LoCP-HiDRUP and LoCP-HiDRUP + Met diets. This, combined with slight differences in milk N production, caused environmental efficiency values to decrease (i.e., less N excreted per kilogram of N in milk) with the LoCP-HiDRUP and LoCP-HiDRUP + Met diets. Urine, milk, and retained N as a percent of absorbed N were not significantly different, but these values also show a trend for milk N as a percent of absorbed N to be higher for the LoCP-HiDRUP and LoCP-HiDRUP + Met diets (P = 0.14).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSStatistical AnalysisRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSStatistical AnalysisRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Received for publication August 12, 2002. Accepted for publication September 26, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSStatistical AnalysisRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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), HiCP-HiDRUP (
), LoCP-HiDRUP (•
) and LoCP-HiDRUP + Met (
). Solid symbols represent the primiparous cows; open symbols are multiparous cows. The solid line represents the regression equation for primiparous cows (NA = 0.80(±0.005) * NI - 103(±17)). The dashed line represents the regression equation for multiparous cows (NA = 0.80(±0.005) * NI - 132(±11)).