Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2008
Publication Date: 9/9/2008
Citation: Lee, M.R., Tweed, J.K., Scollan, N.D., Sullivan, M.L. 2008. Ruminal Micro-Organisms do not Adapt to Increase Utilization of Poly-Phenol Oxidase Protected Red Clover Protein and Glycerol-Based Lipid. Journal of the Science of Food and Agriculture. 88(14):2479-2485. Interpretive Summary: Polyphenol oxidase (PPO), an enzyme present in the tissues of many plants, carries out oxidation reactions that are often associated with browning seen in fresh produce. Leaves of red clover have particularly high levels of PPO. Previous studies have demonstrated that in red clover, these high PPO levels have beneficial effects when the forage is used as feed for ruminant animals. First, PPO prevents protein losses during storage. Second, the protein is less degraded by rumen microorganisms and more efficiently utilized by the animal. These have positive economic (farmers can spend less on protein supplements) and environmental (more efficiently utilized protein means less nitrogen waste is released into the environment via animal waste) impacts. Finally, PPO present in red clover prevents breakdown of fatty acids during storage of the forage and by microbes in the animal’s rumen, leading to increased levels of beneficial polyunsaturated fatty acids in the resulting animal products (milk and meat). The experiments presented here address whether the microorganisms present in the cow’s rumen can adapt to high-PPO forages like red clover to better utilize PPO-protected protein and lipid, and thus diminish the beneficial effects of high-PPO forages. Rumen microorganisms from dairy cows fed red clover (high-PPO) or grass (low-PPO) diets did not differ in their ability to utilize PPO-protected protein or lipid, indicating that they do not adapt to high-PPO diets. Thus, the significant economic, environmental, and human health benefits of feeding ruminants high-PPO forages will not be diminished by adaptation of rumen microbes to such diets. These results suggest that high-PPO forages should be developed, either by conventional breeding or by genetic modification, in order to fully take advantage of PPO’s benefits.
Technical Abstract: The enzyme, polyphenol oxidase (PPO), reduces the extent of proteolysis and lipolysis within red clover fed to ruminants with subsequent increases in the efficiency of N utilization and the level of beneficial polyunsaturated fatty acids in their products (meat and milk). It has also been reported that red clover feeding alters the rumen microbial population compared to grass feeding. This study investigated whether the observed shifts in the microbial population of the rumen when ruminants are fed red clover silage (RC) as opposed to grass silage (G) represented an adaptation by the micro-organisms to increase the utilization of PPO-protected protein and glycerol-based lipid. The experiment consisted of two periods where ruminally fistulated dairy cows were offered either RC or G for two weeks, followed by collection of rumen fluid, which was then used in in vitro incubations to investigate lipolysis and proteolysis over time in plant material derived from red clover plants with either wild type PPO expression (PPO+) or PPO expression reduced to undetectable levels by gene silencing (PPO-). Proteolysis and lipolysis (P < 0.05) were lower after 24 h of incubation in the PPO+ treatment than the PPO- treatment, irrespective of rumen fluid. Biohydrogenation of C18 polyunsaturated fatty acids was also lower on the PPO+ treatment than the PPO- treatment, with no effect of rumen fluid. These results suggest that microbial changes to red clover feeding did not result in an increased ability of the micro-organisms in the present study to utilize either PPO-protected protein or glycerol-based lipid.