Examining the Microbial Basis of Forage Digestion Efficiency in Beef Cattle

Background

Improving feed efficiency can help reduce both feed costs and the environmental impact of fed cattle. There is a large variation between animals when it comes to feed efficiency. Residual feed intake (RFI) is one way to measure efficiency in cattle, but it doesn’t work as well on grazing animals. RFI is also influenced by diet and stage of production, resulting in re-ranking throughout the production cycle due to these factors. These researchers wanted to better understand why some animals are more efficient at digesting forages than others.

Objectives

•  Determine if the microbial ecology of the rumen and/or hindgut varies between animals that differ in forage digestion efficiency
• Identify key genes and/or microbes in high efficiency animals that are not present in low efficiency animals
•  Determine if animal physiology and host metabolism is a bigger factor in feed digestion efficiency than the microbial processes in the gut
• Determine if efficiency rankings in animals change as a result of changes in feed composition and how this might relate to animal and/or microbial factors

What they Did

This team conducted two detailed animal studies (metabolism trials) to understand why some heifers were better than others at digesting fiber despite being on the same diet. In the first trial, they kept all eight heifer on a high-forage diet and measured how much they ate, how well they digested it, how much methane they produced, and how active the microbes in their gut were. The the second trial, they tested each of these animals on four diets – two high in forage and two high in grain – to see how diet affected digestion efficiency, fed intake and the gut microbiome (the microbes that live in the digestive tract, which are important for feed digestion, especially forages, and animal health). The research team collected samples of feed, feces, urine, and rumen contents, and monitored chewing behavior, fermentation patterns, and microbial communities in the rumen and feces. They then used DNA and RNA sequencing to understand which microbes were present, what enzymes they were producing, and to see if this was affecting the genes produced in the gut. The overall goal was to determine if the ability to digest fiber is determined solely by the animal itself, by the microbes in the gut, or a combination of both.

What They Learned

The results showed that heifers classified as ‘efficient’ on the basis of their ability to digest forage generally ate more, digested their feed more efficiently, and tended to gain more weight, even when their diet changed. Surprisingly, these animals also produced less methane per unit of feed. When the team looked at the microbes in the rumen, they found only small differences in microbial populations between animals classified as high and low based on their ability to digest fibre. This suggests that gut microbes alone may not explain why some animals are more efficient. In some cases, we saw shifts in particular groups of microbes, but these differences were usually minor and varied depending on the diet.

Instead, they saw bigger differences in how the animals behaved. High digestibility cattle spent more time chewing and ruminating, which likely helped them break down fiber more thoroughly. They also had larger rumen volumes and may have kept their feed in the rumen longer, giving microbes more time to work. These physical and behavioral traits may play a bigger role in fibre digestion efficiency than the microbial community itself.

The research team also tested new DNA sequencing approaches to study rumen protozoa and fungi, microbes from the rumen that are less well understood than bacteria. These early findings revealed that the protozoa and fungi populations were more related to fibre digestibility ranking than the bacteria and archaea, and suggest that these types of microbes could be a promising future target for improving feed efficiency.