Ruminating on Feed Efficiency and Digestive Physiology

Ontario maintains the highest corn consumption rate of all provinces in Canada, using approximately 4.1 million tonnes of corn grain for livestock feed, part of which contributes to feeding 775,000 head of cattle. With feed prices rising in the past decade and beef producers spending 75% of total costs on feed to produce finished beef, the need for producers to minimize costs while maintaining production is increasing. Selecting for a more feed efficient herd is a valuable opportunity for producers to reduce feed costs through reducing feed inputs. However, it is well known that incorporating feed efficiency into management strategies is impractical on-farm, since the common measure (residual feed intake; RFI) remains an expensive and time consuming method.

To try to solve this problem, researchers study "biomarkers" which are biological measures in an animal that can indicate other measures (ie. feed efficiency). So how do we study biomarkers and their relationships with feed efficiency? It is known that energy requirements differ between feed efficient and inefficient cattle; similarly, some cattle demonstrate higher productive performance than others when fed the same diet and housed in the same environment. This suggests there are individual differences in how efficiently cattle convert their feed to energy used for daily functionii. From this, we studied rumen parameters (Figure 1) to better understand the biological workings behind the rumen and its relationship with feed efficiency and digestive health to discover ways to improve performance.

Rumen parameters and measurements.

Figure 1. Rumen parameters and measurements. Left image showing rumen papillae under microscope. Upper-middle images showing rumen fluid being prepared for short chain fatty acid profile analysis. Right image showing microscopic image of rumen bacteria. Lower-middle image showing rumen probe to measure ruminal pH.

The Experiment

We used 48 crossbred beef cattle fed a high grain diet (78% high-moisture corn), which were separated into efficient (low-RFI) and inefficient (high-RFI) groups. Using a probe (Figure 1, bottom image) placed in the rumen sac, we recorded ruminal pH for 5 to 9 days (Figure 2A). At slaughter, digestive organs were removed and probes were collected (Figure 2B). Rumen fluid was also collected for microbial analysis and short chain fatty acid profiling (Figure 2C). Rumen tissue was collected from three sacs of the rumen and was prepared for microscopic analysis (Figure 2D).

A: Cattle during ruminal pH recording. B: Digestive organs removed at slaughter. C: Rumen fluid sample. D: Tissue from rumen sacs being prepared for microscopy.

Figure 2. A: Cattle during ruminal pH recording. B: Digestive organs removed at slaughter. C: Rumen fluid sample. D: Tissue from rumen sacs being prepared for microscopy.

Ruminal pH

Ruminal pH is a measure of acidity of the rumen fluid and an indicator of overall health and digestive function. It is influenced by factors such as diet composition, microbial diversity, and nutrient absorption rate (Figure 1). The probe recorded ruminal pH continuously every 5 minutes. After slaughter, the data were organized to determine average time spent throughout the day within specific pH intervals defined by digestive condition (Table 1). Data revealed that cattle spent approximately 98% of their time above the acidosis pH interval, indicating healthy digestive function within the herd.

Table 1. Time (%) spent during a day within pH intervals of efficient and inefficient cattle.

Rumen condition pH interval Efficient (% time/day) Inefficient (% time/day)
Acidosis pH pH < 5.59 2.43 2.25
High-grain diet reference 5.60 < pH > 5.99 4.68 5.88
Optimal digestion 6.00 < pH > 6.39 12.27 14.20
High pH pH > 6.40 77.59 76.67

We also developed a 24-hour profile of rumen pH, which was compared between efficient and inefficient animals (Figure 3). There were no differences in the average time spent in pH intervals or 24-hour pH variation between efficient and inefficient cattle. The pH profiles displayed a variation in pH through the day. This is shown by drops in pH after feeding events due to increased acid production from fermentation (Figure 3).

Graph showing Rumen pH on left hand side staring at 6 and rising to 8 at the top increasing by 0.5 each time. The hour of day is shown along the bottom axis starting at 1 on the left and going to 23 on the right. Two lines are drawn from left to right showing efficient and inefficient.

Figure 3. Average hourly ruminal pH over 24 hours compared between efficient and inefficient cattle.

Analyzing the Microbiota

The microscopic organisms living in the rumen represent the rumen "microbiota". The diversity in these microbial species is remarkable and unique to each animal. The microbiota is responsible for fermenting feed components including carbohydrates, proteins, fats, and other organic compoundsi. Microbial analysis consisted of measuring the concentration of microbial species (bacteria, protozoa, methanogens) per mL of rumen fluid. Each microbial species performs different processes during feed digestion and produces different end products during fermentation, and may be different between cattle of divergent feed efficiency.

Short Chain Fatty Acid Profiles

Rumen fermentation produces end-products including short chain fatty acids which are absorbed across the rumen wall through papillae for energy use. Since these fatty acids account for up to 75% of daily energy supply for cattle, we used gas chromatography to measure the total concentration of all short chain fatty acids, as well as concentrations of the major fatty acids: acetic, propionic, and butyric acid. Our research revealed that more feed efficient beef cattle had higher total short chain fatty acid concentration compared to less feed efficient cattleiii, suggesting that efficient cattle digest their feed more efficiently, producing more products for energy use from fermentation.

The Rumen Epithelium

The rumen epithelium is the tissue surface lining the rumen. It projects leaf-like structures called papillae (seen in Figure 1D); this epithelium goes through remarkable changes in structure to absorb energy sources, including short chain fatty acids from fermentation. We measured the epithelial thickness of papillae from the largest rumen sac, a measurement which is represented by the distance from the outer layer (corneum layer) to the middle (connective tissue) (Figure 4, right image). Preliminary studies did not show a difference in papillae thickness between feed efficient and inefficient cattle.

Upper-left image of rumen papillae at 40X magnification. Right image of where 4 areas of the papillae epithelium thickness were measured on the papillae at 100X magnification. Lower-left image at 100X magnification of corneum layer measurement.

Figure 4. Upper-left image of rumen papillae at 40X magnification. Right image of where 4 areas of the papillae epithelium thickness were measured on the papillae at 100X magnification. Lower-left image at 100X magnification of corneum layer measurement.

Current Research

Since feed efficiency was associated with total short chain fatty acid concentration, we are continuing research to investigate further associations between feed efficiency and additional biological parameters. We are currently assessing papillae width thickness on papillae from other sacs of the rumen, as well as the corneum layer thickness (Figure 4, bottom-left image) as previous research has shown that forage-fed cattle have a thicker corneum layer compared to concentrate-fed cattle. Additionally, microbiota analysis and short chain fatty acid profiling is in progress on rumen fluid of heifers fed a forage diet, allowing us to compare the microbiota of cattle fed concentrate and forage diets in the context of feed efficiency.

Implications

Our research will expand our understanding of the biology of feed efficiency and rumen parameters and allow us to investigate digestive health screening methods to determine ways to engineer the rumen for improved function in areas such as feed digestion, digestive disease prevention, and greenhouse gas emissions.

Acknowledgements

We acknowledge the financial support from Agriculture Adaptation Council, Agriculture and Agri-Food Canada, Beef Cattle Research Council, Beef Farmers of Ontario, and Ontario Ministry of Agriculture, Food and Rural Affairs.

References

iHungate, R.E. 1966. The rumen and its microbes. Academic Press. New York and London.

iiLam, S.L., Munro, J.C., Cant, J.P., Guan, L.L., Steele, M.A., Miller, S.P., Montanholi, Y.R. Associations of rumen structure, function, and microbiology with feed efficiency in beef cattle. Book of Abstracts of the 66th Annual meeting of the European Federation of Animal Science, Warsaw, Poland 31 August - 4 September. Pg. 123.

iiiZebeli, Q., Khaiosa-ard, R. 2014. Cattle`s variation in rumen ecology and metabolism and its contributions to feed efficiency. Livestock Science. 66-75.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca