Weather Resilience in Grazing Beef Heifers with Differing Residual Feed Intake
Creators: Londono Mendez, Maria Camila
Publisher: name: University of Alberta Library
Year: 2024

<p>The beef industry seeks sustainability through improved feed efficiency, yet the interaction between residual feed intake (RFI) and weather resilience in grazing beef heifers remains underexplored. This thesis evaluated rumen temperature (RT), blood parameters, growth performance, and activity behavior of beef heifers with varying RFI during summer and winter seasons in Western Canada. Forty-one crossbred beef heifers [351 ± 40 kg body weight (BW); 14 mo of age] were classified as more (LOW-RFI; n = 21, -0.96 ± 0.70) or less (HIGH-RFI; n = 20, 1.4 ± 1.00) feed-efficient. Heifer behavior was monitored using accelerometer-based sensors and individual virtual fence collar technology. Rumen temperature and water access events frequency were monitored using a bolus device, and blood samples were collected to analyze parameters related to protein, lipid, and growth metabolism, as well as neurotransmitters and hormones associated with metabolic homeostasis. The neutrophil-to-lymphocyte ratio (N:LR) was also assessed as a marker of inflammation. Environmental conditions were estimated by calculating the Climate Comprehensive Index. In summer, LOW-RFI heifers tended to exhibit greater free triiodothyronine levels (P = 0.08) and gamma-aminobutyric acid (GABA; P = 0.01) when weather was classified to impose moderate stress. LOW-RFI heifers also tended to produce more heat shock protein 70 (P = 0.10), while HIGH-RFI heifers had greater haptoglobin (P = 0.02) and leptin (P =0.04) concentrations. The RT was greater in HIGH-RFI heifers during specific hours of the day (P = 0.002). For activity behavior, HIGH-RFI heifers had a greater number of steps on certain days, sought shade and water spots during hotter days, and had more daily transitions. In winter, greater leptin concentrations were found in HIGH-RFI heifers (P = 0.04) while GABA tended to decrease in HIGH-RFI (P = 0.08). The LOW-RFI heifers had higher RT on certain days (P = 0.009), more standing activity during moderate cold stress, and higher N:LR on the coldest days. The HIGH-RFI heifers stood more on extreme cold days but had fewer steps on mild cold days and more steps on severe cold days. LOW-RFI heifers had fewer daily transitions. Blood parameters were significantly affected by day, RFI and RFI × day interaction in both seasons (P &lt; 0.001), but there was no significant difference in growth performance between LOW and HIGH-RFI heifers (P &gt; 0.24). Feed-efficient beef heifers in Western Canada exhibited greater resilience to extreme weather conditions, showing variations in blood parameters and rumen temperature (RT) without affecting growth performance. This study identified distinct behavioral and physiological responses between LOW-RFI and HIGH-RFI heifers. LOW-RFI heifers displayed enhanced thermoregulatory activity during both summer and winter, although chronic cold exposure may have potentially negative consequences. These findings explain the potential of selecting LOW-RFI heifers, leading to environmental resilience and sustainable beef production. However, further research is necessary to understand the long-term impacts of chronic cold stress on LOW-RFI heifers.</p>