Research

Background

Previous work has shown that five main physiological processes are likely to contribute to differences in feed efficiency: feed intake/digestion, metabolism, physical activity, and thermoregulation. Pinpointing the exact contribution of these processes has been somewhat more challenging.

While some research has occurred in the United States examining the effect of heat stress on feed efficiency, there is a lack of information in Canada regarding the interaction between feed efficiency and environmental conditions, particularly during summer and winter grazing where cattle may be exposed to more severe weather.

Objectives

The objectives of this study are to:

• Reveal the relationship between cow weather resilience and feed efficiency by assessing cow physiological status, growth performance, immune status, and behavioural responses during summer and winter grazing.
• Associate environmental conditions using equations that account for air temperature, wind speed relative humidity and solar radiation exposure with changes in cow physiology, behaviour, immune status, and reproductive success .

What they did

Feed efficiency testing via GrowSafe® was performed on 49 crossbred heifers at the University of Alberta’s Roy Berg Kinsella Research Station for 80 days. The heifers were fed a standard diet consisting of barley silage and oats. After data cleaning, 23 heifers were classified as low RFI and 21 as high. Then the heifers were managed in one group for both summer and winter measurements (grazing during July-August in summer and fed hay January-March in winter). Forage quality and environmental conditions were measured every two weeks during the sampling periods. A comprehensive climate index (CCI) was calculated for each period using air temperature, wind speed, solar radiation and relative humidity. Weighing and blood sampling also occurred every two weeks. Ultrasound back fat was measured at the beginning and end of the trial. Continuous measures included rumen temperature via rumen boluses, and behavioural activity via pedometers.

What they learned

There were times during both periods that cattle would be under moderate to severe environmental stress according to the CCI. Residual feed intake (RFI) values ranged from a low (efficient) of -2.29 lbs DM/day to a high of 2.4 lbs DM/day. The average for the low-RFI group was -0.96 lbs DM/day and 1.40 lbs DM/day for the high-RFI group. No significant differences existed in animal performance between the two groups in either summer or winter. Both groups of heifers spent similar amounts of time lying down, but the high-RFI heifers spent more time seeking shade and water spots during hotter days, had more daily transitions (from lying down to standing and vice versa), and had higher rumen temperature. High-RFI heifers also demonstrated lower levels of free triiodothyronine (a thyroid hormone) in the blood during summer, which could indicate an adaptation to reduce basal metabolic rate under heat stress.

During winter, low-RFI heifers exhibited higher rumen temperature and more standing activity during moderate cold stress, while high-RFI heifers took more steps on severely cold days. Also, low-RFI heifers had higher lymphocyte counts,  which may indicate differences in immune response and function between efficient and less efficient animals. Interestingly, while no differences existed between groups in summer, leptin concentrations were significantly decreased in low-RFI heifers during the winter, which may be another metabolic adaptation.

What it means

There are physiological and behavioural differences between efficient and less efficient animals in the face of extreme weather conditions, both hot and cold. While these differences may start to explain some of the underlying mechanisms contributing to cow efficiency, this was a preliminary experiment, and more research will be required to determine the practical on-farm implications.