The Fatty Acid Profile of Rye and Annual Ryegrass Pasture Changes during their Growth Cycle

The Fatty Acid Profile of Rye and Annual Ryegrass Pasture Changes during Their Growth Cycle

Sharon J. Freeman-Pounders, Graduate Research Assistant, Animal and Veterinary Science Department, Clemson University, Clemson, SC 29634; Dennis W. Hancock, Forage Extension Specialist, University of Georgia, Athens, GA 30602; Jean A. Bertrand, Assistant Dean for Academic Affairs, College of Agricultural and Environmental Sciences, University of Georgia, Athens, GA 30602; Thomas C. Jenkins, Professor, Animal and Veterinary Science Department, Clemson University, Clemson, SC 29634; and Bruce W. Pinkerton, Assistant Director, College of Agriculture, Forestry, and Life Sciences, Clemson University, Clemson, SC 29634

Conjugated linoleic acid (CLA) is a specific fatty acid (FA) derived from biohydrogenation of linoleic (C_18:2) and linolenic (18:2) acids in ruminant animals (2). By consuming dairy and beef products high in CLA content, humans could obtain dietary concentrations of CLA that have demonstrably reduced the incidence of cancer in animal analogues (2). The concentration of C_18:2 and C_18:3 in forage grasses is known to vary widely between and within species (1), and such changes in the pasture or feed affect CLA synthesis in the ruminant animal and the CLA concentration in ruminant products (2). Information on this variability may influence feeding strategies that optimize CLA concentration in products from pasture-fed ruminants. Therefore, the objective of this study was to document changes in total FA and CLA precursors in two common winter annual forages, rye (Secale cereale) and annual ryegrass (Lolium multiflorum Lam), during their growing seasons.

Dairy paddocks at the LaMaster Dairy Center, Clemson University, Clemson, SC, were conventionally prepared and planted in October 1999 to either rye (cv. ‘Wrens Abruzzi’) or annual ryegrass (cv. ‘Marshall’) at seeding rates of 110 lb and 25 lb/acre, respectively. These pastures were periodically grazed at a medium vegetative stage and mowed to remove refused/mature forage as part of a separate experiment (3). Commercial fertilizer was applied at planting at a rate of 25 lb of N, 70 lb of P₂O₅, and 70 lb of K₂O per acre, in accordance with soil test recommendations. In March 2000, an additional 60 lb of N per acre was applied.

During the 1999-2000 growing season, five forage samples were collected (2-inch stubble height) weekly from random locations within the rye and annual ryegrass paddocks during their grazing periods (November 18 to March 17 and March 31 to June 3, respectively) using repeated measures in a completely randomized design. Samples were flash-frozen with liquid nitrogen and freeze-dried before being ground (2 mm). The FA profile of the forages was assessed by direct methylation of 300 mg of prepared sample and performing FA methyl ester analysis by gas chromatography (4). Data were analyzed using ANOVA and orthogonal polynomial contrasts in SAS (SAS Institute Inc., Cary, NC).

Total FA concentration decreased linearly (P < 0.05) in the rye forage from 6.8% in mid-November to 4.7% in mid-March, but FA exhibited a quadratic decline (P < 0.01) from 4.5% in mid-March to 1.8% in early June in annual ryegrass. Similar to the trend in total FA concentrations, the mean concentration of C_18:2 as a percentage of total FA decreased linearly (P < 0.01) from 58.9% to 41.2% and from 55.4% to 31.6% in the rye and annual ryegrass forages, respectively. The concentration of C_18:2 in the total FA of rye did not change with time but increased from 8.5% in May to 24.5% in early June in the annual ryegrass in a cubic relationship. Since this study represents one year’s data at one location, concentrations and trends should be interpreted cautiously. However, this study showed that the total FA levels and concentration of C_18:3 and C_18:2 in rye and annual ryegrass changed during their growing seasons. Such changes may need to be accounted for in feeding strategies that maximize CLA content.

Literature Cited

1. Dewhurst, R. J., Scollan, N. D., Youell, S. J., Tweed, J. K. S., and Humphreys, M. O. 2001. Influence of species, cutting date and cutting interval on the fatty acid composition of grasses. Grass Forage Sci. 56:68-74.

2. Dhiman, T. R., Nam, S. H., and Ure, A. L. 2005. Factors affecting conjugated linoleic acid content in milk and meat. Crit. Rev. Food Sci. Nutr. 45:463-482.

3. Freeman, S. J. 2004. Fatty acid profile of forages and milk from cows that graze. M.S. thesis. Clemson Univ., Clemson, SC.

4. Sukhija, P. S., and Palmquist, D. L. 1988. Rapid method for determination of total fatty acid content and composition of feedstuffs and feces. J. Agric. Food Chem. 36:1202-1206.