Heart Rate: A Potential Tool for the Improvement of Feed Efficiency and More

Times are Good in the Beef Industry… Right?

From the outside looking in, one might say "Must be good to be a beef farmer with all these high prices." Most cow-calf producers would agree. However, for farmers running feedlots and backgrounding operations, the outlook may not be quite as "rosy". These sectors operate on a margin, with the purchase cost of stockers and feeders representing a large proportion of operating costs. If these farmers were asked today, many would agree that this margin has not increased. So then how can beef farmers take advantage of high market prices without a parallel increase in their cost of production? A potential answer is by improving the feed efficiency of their cattle. Residual feed intake (RFI; measure of feed efficiency) is able to identify cattle that maintain gains and carcass composition while reducing feed intake. However, the determination of RFI requires the recording of individual intake, an expensive venture. Therefore, there is the need for alternative means of assessing feed efficiency on-farm. Our research group is working to develop heart function as a potential tool for meeting this need.

Making the Connection: From Heart Function to Feed Efficiency

The heart functions to deliver oxygen and nutrients to tissues of the body where they are used to supply the energy that is required for the animal to live, respond and perform. As this energy requirement changes so too does the heart's function. Energetic requirements are different between feed efficient (low-RFI) and feed inefficient (high-RFI) cattle, suggesting that heart function could also vary with RFI. A preliminary study from our group has shown that heart rate was different between low-RFI and high-RFI bulls during rest, transport and while at an abattoiri.

Heart Control: the Role of the Nervous System

Before a method for using heart rate on-farm is developed, the control of the heart must be understood. The heart is controlled by two branches of the central nervous system (Figure 1), the:

  1. parasympathetic nervous system (PNS)
  2. sympathetic nervous system (SNS)

Heart control. Suggested route of action for a change in heart rate by changing the energy requirement of cattle. [O2: oxygen level, CO2: carbon dioxide level, BP:blood pressure]

Figure 1. Heart control. Suggested route of action for a change in heart rate by changing the energy requirement of cattle. [O2: oxygen level, CO2: carbon dioxide level, BP:blood pressure]

Text equivalent of Figure 1

Sensors located near the heart detect levels of oxygen, carbon dioxide and blood pressure. These sensors send signals to the brain (hypothalamus) which compares the signal to a set point that depends on a variety of factors including energy requirements. If oxygen levels are determined to be too low for the current energy requirement the PNS is inhibited and the SNS stimulated, causing release of catecholamines that act to increase heart rateii. As RFI is also related to energetic requirements, heart rate changes could be different between low- and high-RFI cattle.

Study 1: On-Farm Heart Rate Assessment to Screen for low-RFI Cattle

This study is developing a heart rate assessment that could be used on-farm to screen for feed efficient cattle. Our objective is to cause short term changes in energy requirements so that a difference in heart rate response occurs between low- and high- RFI cattle.

Photo of the inside of a barn. A beef animal is held in a squeeze shoot. A man is shown on the left side of the photo holding an open umbrella.

Figure 2. Heart rate assessment technique, equipment and facilities used in Study 1 at the Maritime Beef Test Station.

To cause this short term change in energy requirement, heifers of known feed efficiency were restrained in a squeeze chute and equipped with a heart rate recording harness (Figure 2). Heart rate was monitored prior to, during and after an umbrella was opened and closed in front of the heifer's snout (Figures 2 and 3).

Heart rate monitoring protocol and traits assessed. HRBEFORE; average heart rate before umbrella, HRAFTER; average heart rate after umbrella, HRINCREASE; heart rate increase: HRAFTER - HRBEFORE.

Figure 3. Heart rate monitoring protocol and traits assessed. HRBEFORE; average heart rate before umbrella, HRAFTER; average heart rate after umbrella, HRINCREASE; heart rate increase: HRAFTER - HRBEFORE.

Text equivalent for Figure 3

Preliminary Results

Low-RFI heifers had a greater average heart rate after the umbrella (HRAFTER) and a greater increase in heart rate due to the umbrella (HRINCREASE). This suggests that low-RFI heifers may have a more responsive SNS causing an increase in heart rate when energy requirements are increased. As high-RFI cattle have higher energy requirements their energy levels could have been high enough before the umbrella to accommodate a response without an increase in heart rate.

Study 2: The Underlying Biology: Understanding the Heart Rate-RFI Relationship

An ongoing research trial at the University of Guelph is attempting to further develop heart rate as an assessor for feed efficiency by diving into the biology of the cardiac system. At the Elora Beef Research Centre (EBRC), yearling bulls and steers of known feed efficiency were equipped with a heart rate monitoring harness (Figure 4) and heart rate was recorded during rest, handling, transport and at an abattoir. Stroke volume, the amount of blood the heart pumps per beat, was assessed on all cattle (Figure 5).

Two beef bulls wearing recording harnesses, one standing and one lying down.

Figure 4. Yearling steer equipped with heart rate recording harness during Study 2 at the Elora Beef Research Centre.

At slaughter, heart tissue was collected and the heart was dissected (right ventricle, left ventricle, atria and pericardial fat) and weighed. Tissues of the heart will be assessed at both the histological (cellular) and molecular level (Figure 5) to determine if differences in heart cellular structure and receptor abundance occur between cattle of differing feed efficiency.

Biological assessments done, in progress or remaining on yearling steers and bulls in Study 2. All biological assessments have a tie to heart function.

Figure 5. Biological assessments done, in progress or remaining on yearling steers and bulls in Study 2. All biological assessments have a tie to heart function.

Text equivalent to Figure 5

Preliminary Results

Low-RFI cattle had a heavier right ventricle which is associated with increased sympathetic activity, supporting the SNS hypothesis proposed in Study 1.

Implications

With continuing development heart rate assessment could be used on-farm to screen for feed efficient cattle. The in-depth biological understanding of the heart rate-feed efficiency relationship will ensure both solid development of this potential technology and will assist in minimizing the potential undesirable outcomes that could arise from screening for feed efficient cattle with heart rate.

Bringing it to the Farm

Upon continued development the heart rate assessment could be used during routine handling practices on farm such as:

  • weaning
  • cattle processing
  • any time cattle are passed through a squeeze chute

The earlier in life the assessment is done, the earlier feed efficient cattle can be screened, the more feed costs can be reduced and the more the farmer's margin can be increased.

There's More!

Recent research in cattle has observed heart rate to also be an indicator of:

  • temperament, welfare and behaviour
  • handling and pathological stress
  • a potential indicator of time till calving and calving onset

Taken all together, these new studies may lead to heart rate being used on-farm as a versatile management tool.

References

i Montanholi Y.R., Lam S., Miller S.P. 2014. Heart rate assessment in beef bulls prior to slaughter is associated with feed efficiency. Book of Abstracts of the 65th Annual Meeting of the European Association for Animal Production, Copenhagen, Denmark 25-29 August 2014. Pg. 259.

ii Purves, D., Augustine, G., Fitzpatrick, D., Katz, L., LaMantia, A., McNamara, J., Williams, S. (Eds.), 2001. Neuroscience, 2nd ed. Sinauer Associates.


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