The impact of genomic selection for feed efficiency on the cow-calf sector, performance parameters and underlying biology
Advances were made in the area of assessing and monitoring biological responses in cattle with divergent feed efficiency. Improving feed efficiency is crucial to the sustainability of the beef industry. However, the direct assessment of feed efficiency in the bovine is not practical for commercial operations due to the duration of the assessment and equipment and labor costs. Moreover, the “blind” selection for feed efficiency may lead to deleterious correlated genetic selection responses. Therefore, efforts were made to identify novel parameters that could be used to indirectly identify feed efficiency, and to monitor responses to improved feed efficiency in beef heifers and bulls. More specifically, four streams of research were advanced, including: blood hematology and metabolomics in replacement beef heifers, and heart function, rumen physiology and fertility related traits in young beef bulls. The hematological study in heifers also contributed to the understanding of estrus biology and detection.
Feeding expenses and reproductive success are major drivers impacting profitability in the cow-calf sector. Similarities between feed efficient phenotypes and estrus state may be supported by markers of energetic shifts. Hematological measures associated with energy metabolism were investigated as proxies of feed efficiency and estrus in forage-fed beef heifers. Experiment 1 evaluated blood cell parameters, immunoglobulins and metabolites in relation to feed efficiency in heifers; Experiment 2 assessed metabolites to serve as proxies for estrus. Efficient heifers had greater lymphocytes, immunoglobulin M response, and lower alkaline phosphatase (ALP) concentrations. Efficient pregnant heifers had lower concentrations of cholesterol and globulin. Estrus state was strongly associated with fluctuations of ALP, aspartate aminotransferase, beta-hydroxybutyric acid, creatine kinase and triiodothyronine concentrations. However, age, body size and composition may influence such associations. There is potential for hematological parameters to serve as proxies of metabolic shifts related to feed efficiency and estrus state.
Proxies have the potential to accelerate feed efficiency improvement, assisting with reduction of beef cattle feed costs and environmental impact. Heart rate (HR; BPM) is associated with feed efficiency and influenced by autonomic activity and peripheral metabolism, suggesting that HR could be used as a proxy for feed efficiency. Feed efficient heifer calves have a lower overnight heart rate and an increased heart rate upon acute stress. Contrasting acute stress results in yearling heifers suggest that coping styles vary across categories of cattle. Overall, results indicate that overnight and acute stress heart rates are potential proxies for feed efficiency in heifer calves. Overnight HR and acute stress HR are potential indicators of RFI in heifer calves. However, acute stress HR results varied in yearling heifers, suggesting previous handling experience and maturity alter acute stress response. Pending further development (predictive ability), the acute stress assessment could have potential for on-farm application as a feed efficiency proxy in young heifers.
Investigation of rumen physiological characteristics and their association with feed efficiency can provide biological markers for selecting more feed efficient cattle. The rumen microbial community produces nutrients metabolized by the rumen epithelium which maintains metabolic efficiency. Our results showed more feed efficient cattle spend more time in a desirable pH range, have greater papillae thickness, higher bacterial concentration, and lower methanogen concentration compared to less feed efficient cattle. Feed efficient animals therefore have improved feed energy utilization and reduced methane emission. Using biological markers to develop selection programs can reduce production costs and improve environmental conservation of beef production.
Feed efficiency and bull fertility are two major factors affecting profitability of the beef industry. However, there is concern of an antagonistic relationship between these two factors, highlighting the need to clarify the relationship between age, feed efficiency and sexual development. Two experiments were undertaken to evaluate associations of blood parameters and fertility-related measures with age and feed efficiency in yearling bulls. Firstly, bulls were studied and among other results, younger and efficient bulls exhibited lower testosterone and triiodothyronine levels, respectively. Secondly, investigation of young bulls revealed that younger bulls had smaller scrotal circumference, higher scrotal radiant heat loss and fewer normal sperm while efficient bulls indicated lower scrotal circumference, scrotal radiant heat loss, and a trend towards lower testicular echogenicity and higher sperm head defects. Metabolic differences associated with variation in feed efficiency may impact reproductive function as illustrated by features of delayed sexual development in efficient bulls.
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