U. M. Yang
D. L. Palmquist1
The Ohio State University
Department of Animal Sciences
1 For more information contact at: The Ohio State University, Ohio Agricultural Research and Development Center, 312 Gerlaugh Hall, 1680 Madison Avenue, Wooster, OH 44691; 330-263-3795; fax: 330-263-3949; e-mail: palmquist.1@osu.edu
Feed particles in the rumen adsorb fatty acids, decreasing the inhibitory effect of fatty acids on the ruminal bacteria. Procedures were developed to measure the amount of fatty acids adsorbed to feed particles, and researchers asked whether adsorption capacity varies among feedstuffs. Research indicated that alfalfa hay adsorbs twice as much fatty acid as does timothy hay and wheat straw. Particle size of the forage also influenced adsorption; smaller particles adsorbed more fatty acid than larger particles. Forage source can be a factor to influence usefulness of fat in the ration.
On entering the rumen, dietary acyl lipids are subjected to hydrolysis by microbial lipases, and free fatty acids (FFA) are liberated. These FFA are utilized to some extent by rumen microorganisms for microbial lipid synthesis (Demeyer et al., 1978; Harfoot and Hazlewood, 1988; Hawke, 1971), but FFA concentrations above certain levels may inhibit the growth of rumen bacteria (Henderson, 1973; Maczulak et al., 1981) and ruminal fermentation (Chalupa et al., 1984, 1986). With respect to the nutritional function of the lipids, it is well known that dietary fat has an inhibiting effect on ruminal fermentation and digestion of the diet. Recently, it also was shown that the lipid content in grass correlates negatively with ruminal protein degradability (Fujita et al., 1991), and that grass lipids may inhibit the growth and activity of rumen fibrolytic bacteria (Hino and Nagatake, 1993; Yang and Fujita, 1996). Harfoot (1981) showed that most of the FFA in the rumen is present in association with the food particles, if for no other reason than that these particles are mostly hydrophobic and constitute by far the largest proportion of the solid material within the rumen.
Feed particles compete with microbial surfaces for adsorption of fatty acid and thereby reduce their inhibitory effect on the microbes (Harfoot et al., 1974; Maczulak et al., 1981). Further, limited evidence from feeding studies (Ameny et al., 1995; Ben Salem et al., 1993; Smith et al., 1993), suggests that forages differ in their effectiveness in maintaining optimum rumen conditions when supplemental fat is fed. We postulate that differing capacities of forages to adsorb fatty acids to their surfaces contribute to these differences. It is important, therefore, to know how much fatty acid can be adsorbed to plant particles. This study was conducted to develop appropriate methods and techniques to measure the adsorption of fatty acid to plant surfaces and to investigate the capacity of fatty acid adsorption to particles of different sizes from varying sources and types of forages.
Three different forages, alfalfa hay, timothy hay, and wheat straw as representative samples of legume, grass, and low-quality roughage, respectively, were taken from The Ohio State University's Ohio Agricultural Research and Development Center (OARDC) farm supplies for preparing different particle sizes as follows. After grinding through a 4 mm screen in a Wiley mill, samples of three different particle sizes were prepared using sieves for 1 and 2 mm pore size. The largest particle size sample (#4) passed through a 4 mm screen and was retained on a 2 mm screen (>2 mm<4mm); the next particle size (#2) passed a 2 mm screen and was retained on a 1 mm screen (>1mm<2mm). The smallest sample (#1) passed a 1 mm screen (<1mm); very fine particles were removed by using a 50 mesh sieve.
Five ml of potassium heptadecanoate (17:0) in 50% ethanol, 4.0 mg/ml, was added to varying amounts (25, 50, 100, and 200 mg) of each particle size from three different forages. The contents in a screw cap test tube were incubated in air at 37ºC for four hours with shaking. After incubation, the tubes were centrifuged for one minute at 1,500 rpm to remove supernatant (unbound fatty acid). Five ml warm water (40ºC) was added, and the sample was vortexed slightly (1-2 sec); the tubes were recentrifuged for one minute at 1,500 rpm, and the supernatant removed (to remove remaining unbound fatty acid). The pellet was washed and vacuum-filtered through Whatman #1 paper with warm water (10 ml) and ethanol (4 ml) and dried overnight at 55ºC. The tubes with sample were maintained at 37ºC from incubation to filtering.
Dried samples were methylated directly using nonadecanoic acid (19:0) as an internal standard according to the method of Sukhija and Palmquist (1988) and quantified by gas-liquid chromatography. The amount of fatty acid adsorbed to the feed particles was calculated after correcting for amounts retained by the filter paper.
Adsorption of fatty acids of each particle size from three different forages is shown in Table 1. The amount of fatty acids adsorbed, without exception, increased with increasing concentration of added fatty acids, and decreased with increasing size of particles, in all forages (alfalfa, timothy, and wheat straw). The amount of fatty acids adsorbed to alfalfa hay was higher, approximately two- or three-fold, compared to the amount bound to timothy hay and wheat straw, respectively. Even comparing #4 particles of alfalfa to #1 particles of timothy, fatty acids adsorbed by alfalfa was higher. Also, adsorption by timothy was higher than by wheat straw.
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Table 1. Adsorption of Fatty Acids by Forages (Alfalfa Hay, Timothy Hay, and Wheat Straw) of Different Particle Sizes. | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Adsorbed FA (mg/g feed) | |||||||||
| Added FA (mg/g feed) | Alfalfa Hay | Timothy Hay | Wheat Straw | ||||||
| #1 | #2 | #4 | #1 | #2 | #4 | #1 | #2 | #4 | |
| 100 | 87.41 | 76.52 | 69.78 | 56.36 | 53.23 | 42.21 | 36.56 | 30.44 | 27.30 |
| (2.42) | (3.14) | (4.30) | (3.91) | (3.02) | (5.37) | (2.35) | (2.77) | (3.92) | |
| 200 | 145.68 | 104.75 | 87.68 | 74.69 | 60.78 | 42.35 | 39.44 | 31.41 | 27.93 |
| (5.77) | (3.87) | (6.54) | (6.03) | (3.55) | (2.25) | (2.84) | (2.62) | (2.18) | |
| 400 | 164.47 | 119.40 | 90.75 | 77.53 | 61.18 | 45.99 | 45.76 | 34.41 | 28.19 |
| (8.05) | (4.54) | (6.74) | (4.98) | (3.03) | (4.02) | (10.11) | (5.13) | (4.78) | |
| 800 | 184.26 | 133.39 | 99.86 | 80.51 | 66.45 | 52.65 | 46.97 | 37.01 | 28.97 |
| (3.84) | (9.65) | (9.41) | (3.56) | (0.78) | (5.77) | (6.34) | (7.91) | (8.24) | |
| #1: >50 mesh<1 mm (50-1) #2: >1 mm<2mm (1-2) #4: >2mm<4mm (2-4) Values are means for three determinations with standard deviation as numbers in parentheses. | |||||||||
A Lineweaver-Burk plot of fatty acid adsorption by alfalfa hay particles of different sizes is shown in Figure 2. The reciprocals of the x and y axes intercepts may be compared as Km (the concentration of fatty acid to achieve one-half saturation), and as Vmax (the maximum amount of fatty acid adsorbed by the particles), respectively. The R2 of each regression equation was very high (#1: 0.9699, #2: 0.9943, and #4: 0.9672), meaning the amount of fatty acids adsorbed depends closely on the amount of added fatty acids. The Vmax (maximum velocity) or maximum adsorption of fatty acids to each particle was 236.9, 150.0, and 105.4 mg fatty acids per gram forage for #1, #2, and #4 particles, respectively.
Smith et al. (1993) suggested that tallow in corn silage diets decreased milk fat percentage, compared to tallow in alfalfa hay diets. Also, Ameny et al. (1995) found that adding fat to diets based on corn silage caused greater rumen fill (lower ruminal digestibility) and lower acetate:propionate ratio than diets with no added fat or high-fat diets based on alfalfa hay, suggesting that fat inhibited digestibility to a greater extent with corn silage than with alfalfa. These research data support the hypothesis that alfalfa in the diet, as compared to corn silage, is a positive factor in feeding nonrumen-inert fats, and that differing capacities of forages to adsorb fatty acids to their surfaces may be a factor. The data showed clearly that the capacities for adsorption of fatty acids to forage particles differed among forages and particle sizes. In this case, alfalfa hay adsorbs the highest amount of fatty acid compared to timothy or wheat straw.
We have also found that alfalfa silage adsorbs more fatty acids than does corn silage. The greater adsorption of fatty acids by alfalfa compared to other forages would decrease the amount of fatty acids available for binding to microbial cell walls, a process which inhibits rumen microbial activity and fiber digestion.
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Smith, W. A., B. Harris Jr., H. H. Van Horn, and C. J. Wilcox. 1993. Effects of forage type on production of dairy cows supplemented with whole cottonseed, tallow, and yeast. J. Dairy Sci. 76:205.
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Yang, U. M. and H. Fujita. 1996. Effect of different source of lipids on ruminal fermentation of grass components and microbial growth in vitro. Anim. Sci. Agric. Hokkaido 38:77. (in Japanese).