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* Centre for Dairy Research, School of Agriculture, Policy and Development, University of Reading, RG6 6AR, United Kingdom
Adisseo, Commentry 03600, France
1 Corresponding author: r.h.phipps{at}reading.ac.uk
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ABSTRACT |
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 TOP ABSTRACT ACKNOWLEDGEMENTSREFERENCES |
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Key Words: dairy cow • methionine supplement • milk protein
The relative inefficiency with which N is used for milk production, coupled with the tendency to overfeed CP to help maximize milk production, can contribute to pollution through increased N excretion (St-Pierre and Thraen, 1999). Although a reduction in N excretion can be achieved by decreasing the amount of dietary CP offered, this is generally associated with a reduction in milk and milk protein yield (Broderick, 2003).
Methionine and Lys have been shown to be limiting amino acids for milk yield and milk protein production (Schwab et al., 1992). When reviewing published data, St-Pierre and Sylvester (2005) noted that although 2-hydroxy-4-(methylthio) butanoic acid (HMB) had been used extensively as a source of supplemental Met in dairy cow diets, the response in terms of milk yield, milk protein, and milk fat varied and suggested that it may have been associated with variability in apparent rumen degradability of HMB. Robert et al. (2001) reported that the isopropyl ester of HMB (HMBi) had bioavailability (as metabolizable Met) values of around 50% in lactating dairy cows. The portion of HMBi not absorbed from the rumen is largely hydrolyzed to HMB, which could improve rumen function, resulting in improved milk composition (Robert et al., 2002).
Results from short-term changeover design trials (Robert, 2005) have shown that milk protein content increased between 0.09 and 0.14 percentage units when cow diets were supplemented with HMBi. In these trials, there were no effects of HMBi on DMI, milk yield, or milk fat content. However, in a continuous design lactation study, HMBi fed at 1.5 g/kg total ration DM did not affect DMI but increased milk yield and milk protein content (St-Pierre and Sylvester, 2005). Although the response in milk protein content was almost immediate, milk yield response was gradual and only reached significance after 11 wk of supplementation. Feeding HMBi numerically increased milk fat concentration, and although the effect was not significant, the increase in FCM observed was greater than the increase in milk yield. In addition, supplementation with HMB and HMBi improved the efficiency of N utilization (St-Pierre and Sylvester, 2005).
From both an economic and environmental standpoint, there is interest in determining whether a decreased dietary CP concentration together with a metabolizable Met supplement would support milk protein production to the same extent as an unsupplemented standard CP diet, thereby improving N utilization. The aim of the current study was to determine the effects of HMBi (Metasmart, Adisseo, Commentry, France) on feed intake, milk yield, and milk composition of Holstein dairy cows receiving a TMR with either a low or standard dietary CP concentration and to ascertain if performance can be maintained when feeding HMBi with lower-protein diets.
The work was conducted under the authority of the UK Animals (Scientific Procedures) Act 1986. Cows were housed in a free-stall barn with sawdust for bedding and with ad libitum access to potable water. Sixteen multiparous Holstein cows (DIM: 72 ± 16, BW: 640 ± 47.8, and milk yield: 45.0 ± 2.95 kg/d) were allocated to a 4 x 4 Latin square experiment with 4 simultaneous independent squares, balanced for carryover effects, and with periods lasting 4 wk to determine the effect of HMBi (0 vs. 1.26 g/kg of total ration DM) and dietary CP content [low protein (LP) vs. standard protein (SP)] on feed intake, milk yield, and composition using a 2 x 2 factorial arrangement giving 4 treatments (LP, LP + HMBi, SP, SP + HMBi). The HMBi was fed as the commercial product MetaSmart Dry (Adisseo), with a minimum guarantee of 95% HMBi monomers, supplied as a 60% premix with silica as an inert carrier. MetaSmart Dry was included in the concentrates at a rate of 4.2 g/kg of concentrate DM. This was equivalent to 2.1 g/kg of total ration DM, given that the ration contained 50% concentrates on a DM basis. This resulted in an HMBi monomer concentration in the total ration of 1.26 g/kg of DM, which provided an extra 11.7 g/d of metabolizable Met. This inclusion rate is very close to that used by St Pierre and Sylvester (2005). All cows received a TMR containing corn silage and grass silage in a ratio of 3:1 (DM basis) and containing a 50:50 mixture of forage:concentrate (DM basis; Table 1
). The CP content of LP diets was 14.7% DM compared with the SP diets of 16.9% DM (Table 1). Before the addition of HMBi, diets were estimated to provide 6.71 and 1.86% Lys and Met, respectively, in metabolizable protein in the LP diet and 6.74 and 1.82% in the SP diet calculated using the feed-into-milk model (Thomas, 2004). Basal dietary CP concentration was altered by varying the inclusion rates for rapeseed meal and rumen-protected (formaldehyde-treated) soybean meal (Sopralin, 52% CP, 40% digestible undegradable protein, DM basis, Trouw Nutrition, Northwich, Cheshire, UK). In wk 4 of each period, daily samples (250 g/sample) of corn silage, grass silage, and the concentrate mixture fed were composited to form a representative sample for each period and frozen (–20°C). Silage samples were analyzed for nutritional characteristics (Eurofins Laboratories, Woodthorne, Wolverhampton, UK). Oven-dried (60°C until static weight) silage samples were analyzed for DM, CP, NDF, starch, and water-soluble carbohydrates, using near-infrared spectroscopy (Foss 5000 NIR Systems, York, UK). The ME concentration for grass silage, corn silage, and concentrate supplement was estimated according to Offer et al. (1996) and Givens et al. (1995). The concentrate supplement was analyzed for DM, CP, NDF, starch, water-soluble carbohydrates, and oil content using wet chemistry methods, and ME content was estimated (Ministry of Agriculture Fisheries and Food, 1993). These results were used to calculate the nutritional content of the TMR offered during the experimental periods (Table 1).
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There was no effect of dietary CP concentration or the HMBi supplement on DMI (Table 2). There was an interaction between dietary CP concentration and the HMBi supplement for milk yield (P = 0.03) and FCM (P < 0.02). Feeding HMBi decreased milk yield and tended to decrease FCM (P < 0.09) when the LP diet was fed but not when the SP diet was fed (Table 2). The interaction between dietary CP concentration and the inclusion of HMBi for milk and FCM yield contrasts with other work (Leonardi et al., 2003), in which there was no significant interaction between dietary CP concentration and Met supplementation. This may have been due to the fact that Leonardi et al. (2003) used not only a different source of Met (rumen-protected DL-Met, Mepron M85, Degussa Corp, Allendale, NJ) but also provided greater dietary CP concentrations (16.1 and 18.8%, DM basis) compared with those used in the current study (14.7 and 16.9%, DM basis). This suggests that when the low-CP diet was fed in the present study, other factors such as the supply of RUP or Lys and (or) other amino acids in metabolizable protein may have been limiting, and not just Met supply. In addition, differences in RDP or fermentable carbohydrates between the LP and SP diets may have influenced the response to HMBi in the present study.
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Neither the current study nor that of St-Pierre and Sylvester (2005) established significant treatment effects of HMBi on milk fat concentration. As already indicated for FCM yield, in the present study, there was a significant interaction between diet CP and HMBi for milk fat yield (P < 0.03; Table 2
), with milk fat yield being greater for SP + HMBi compared with LP + HMBi. This difference in yield of milk fat primarily reflected differences in milk yield, because milk fat concentration was not affected (P > 0.13), and again may reflect an interaction between dietary supply of rumen-degradable substrates and effects of HMBi.
In conclusion, when diets with a standard CP concentration were fed to Holstein cows in early lactation, the inclusion of HMBi (1.26 g/kg of total ration DM) increased milk protein concentration and yield. The interaction between dietary CP concentration and inclusion of HMBi, in which the increase in milk protein yield was not observed with the LP diet due to a decrease in milk yield, suggests that other factors limited the response to HMBi when dietary CP supply was restricted.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTACKNOWLEDGEMENTSREFERENCES |
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Received for publication December 12, 2007. Accepted for publication June 23, 2008.
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  R. S. Ordway, S. E. Boucher, N. L. Whitehouse, C. G. Schwab, and B. K. Sloan Effects of providing two forms of supplemental methionine to periparturient Holstein dairy cows on feed intake and lactational performance J Dairy Sci, October 1, 2009; 92(10): 5154 - 5166. [Abstract] [Full Text] [PDF]
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