One of the articles that I read a while ago was “Maximizing efficiency of rumen microbial protein production” written by T. Hackmann and Jeffry Firkins. The article reignited my interest in the topic of ruminal microbial protein efficiency and its impact on overall production efficiency. Here, I would like to share my thoughts and findings with you. The authors did a great job in discussing the topic despite the obvious lack of data from the ruminant side. I invite you to give it a read. (https://www.frontiersin.org/articles/10.3389/fmicb.2015.00465/full)
The rumen is a magnificent organ that host a very complex microbial eco system, which contains manly bacteria, protozoa, fungi, and archaea. The role of which – form the host’s perspective – is to ferment forages and feedstuffs mainly into short chain fatty acids (SCFA), which in turn constitute most of the energy sources to the host animal. In addition, the microbial cells passing through the rumen to the small intestine provide the major source of AA to the host animal (60-85% of AA reaching the small intestine). The fermentation is a process utilized by microbes to generate the necessary ATP for growth (SCFA is simply a by-product of this process). However, only 1/3 to 2/3 of the generated ATP is directed towards growth and the rest for maintenance functions, synthesis of reserve carbohydrates, or energy spilling (futile cycles where energy is released as heat). As the authors indicated, it is very interesting that the microbial growth is an inefficient process, and under some conditions, only a 1/3 of ATP is directed towards microbial growth. The microbial protein is a significant source of metabolizable protein (MP) to the animal and it ideally should make 50% of total MP (microbial + dietary) reaching the small intestine. The amount of microbial MP can exceed 1 kg per day for high producing cows. Therefore, it is important to consider the factors that can affect the efficiency of microbial growth. The optimization of microbial growth can have a significant effect on farm economy.
The authors, using available data, explained some of the factors that affected microbial growth efficiency, which included: Excess carbohydrate supply, recycling of microbial protein, branched chain-short fatty acids, ammonia-N vs. peptides or AA supply, asynchrony of carbohydrate and nitrogen sources, ruminal SCFA interconversion, and supplemental fat. It is interesting that microbes respond in different ways to excess carbohydrates, one of which is by shifting to catabolic pathways that yield less ATP. In general, rapidly fermentable carbohydrate can lead to a drop in ruminal pH (<6), which inhibits fibre digestion (depressed adherence of cellulolytic bacteria to cellulose and reduced cellobiose transport across membrane).
In conclusion, this review highlights the potential inaccuracies in predicting microbial protein production in the rumen and explains several factors that can influence its efficiency. The authors suggested using mechanistic models (rather than conventional models) to improve the prediction of microbial protein in the rumen, however, there is a significant lack of data and more trials are needed. This is a very exciting opportunity to improve ruminant production efficiency!
Ousama AlZahal
(Image credit: Rigobelo and de Ávila, 2012. DOI: 10.5772/50054)