This paper was chosen by a committee of the Canadian Society of Agronomy (www.agronomycanada.com ) as the best agronomy paper published by the Canadian Journal of Plant Science (www.aic.ca ) in 2009. The full title is “Differential ruminal degradation of alfalfa proteins” by D.Chen, M.D. Peel, K.C. Olson, B.C. Weimer and D.B. DeWald, pages 1065-1074 of volume 89.
Alfalfa has one of the highest crude protein contents among forage crops, but rumen micro-organisms rapidly and extensively degrade it. This decreases the amount of total protein available for livestock nutrition and increases the risk of bloat caused by the excessive build-up of gas in the rumen. Thus, alfalfa protein utilization could be improved by increasing the proportion of bypass proteins, which are those that avoid or have decreased rate of degradation in the rumen.
This alteration would increase metabolizable protein that passes into the small intestine where it can be used directly by the animal. Additionally, increasing the amount of protein resistant to rapid microbial degradation may reduce the bloat danger associated with grazing alfalfa.
Breeding an alfalfa variety with increased bypass proteins requires two things: genetic variation for reduced microbial digestion and the capacity to consistently identify these lines. Alfalfa total protein is composed of several different types of proteins. While the variation among alfalfa varieties for rumen bypass of total protein has been studied, there is very little information on the microbial degradation of specific types of alfalfa proteins. If certain types of proteins that are degraded to a lesser extent than others can be found, selection for varieties with higher concentrations of these proteins may be possible, thereby increasing total bypass protein and reducing the risk of bloat. In turn, alfalfa growers may benefit from the production of a higher value crop.
Research characterizes individual proteins for ruminal digestion
To examine differential protein digestion, three distinct varieties of alfalfa, grown from single plants, were subjected to fermentation in the rumen of a cannulated steer for zero, 45 and 120 minutes. A cannulated steer is an experimental animal that has been surgically fitted with a porthole-like device allowing samples to be inserted and retrieved from the rumen.
After digestion, total protein was extracted from the fermented alfalfa residues and the individual proteins were separated using two-dimensional gel electrophoresis.
The individual proteins were labelled with different fluorescence dyes for identification and quantification. The degradation times were labelled separately and the gels were digitally scanned and processed to determine amounts of the different proteins. Individual proteins were identified by removing the spots from the gels, determining their amino acid sequences and comparing them to amino acid sequences from known proteins.
Twenty-six major proteins ranging from stable to highly digestible were successfully characterized from the alfalfa varieties. These proteins represented 36 percent of the total protein detected. After 45 minutes of rumen digestion, nine of these proteins still had greater than 75 percent of their total mass remaining, 12 had 50 percent or less remaining, and six were intermediate. After 120 minutes of rumen digestion, four proteins still had greater than 80 percent of their mass remaining, seven were between 80 and 50 percent, and 16 had less than 50 percent remaining. After both 45 and 120 minutes of rumen digestion, there were three proteins showing differences among the three alfalfa varieties, while most of the other proteins showed similar rates of digestion. After 120 minutes, there were significant differences in protein digestion rates among the three varieties for 10 out of 26 of the identified proteins.
Our experiment shows that we could improve the value of alfalfa as a dietary protein source by selecting varieties with a higher percentage of the proteins that are digested more slowly, or escape rumen digestion altogether. This will result in a greater concentration of the bypass proteins. These improvements would lead to more metabolizable protein for the animals and reduce the chances of bloat from excessive rumen gas production. Since more of the nitrogen-containing protein will be digested by the animals, there will be less environmental contamination from animal wastes.
To take advantage of this research, a rapid and reliable rumen digestion procedure needs to be refined so that large numbers of varieties can be tested. We also need to determine what the effects are of changing protein types and concentrations on the plant’s agronomic performance. Finally, animal feeding trials must be done to show that the new alfalfa varieties result in greater metabolizable protein and less risk of bloat.
The Canadian Society of Agronomy (CSA) is a not-for-profit professional organization whose members work on the science of crop production in Canada. The CSA provides the agronomy editors for Canadian Journal of Plant Science, which is Canada’s premier peer-reviewed journal dealing with the science of agronomy.
