Blood Profiling to Indirectly Assess Feed Efficiency in Grass-Fed Beef Heifers

Expensive feed costs for beef producers can be minimized by increasing feed efficiency. Improvement in the efficiency of feed utilization by the cow herd can lead to benefits in productive efficiency across all sectors of the industry, since this trait is heritablei. Residual feed intake (RFI) is a measure of feed efficiency that requires the assessment of feed intake, which is too expensive and time consuming to implement on commercial beef farms. However, indirect measures such as a blood component profile may serve as proxies for feed efficiency, due to their associations with energy metabolism. Specific blood parameters include complete blood cell count (CBC), immune system challenge and blood plasma metabolites.

Heifers and Experimentation

A group of 107 crossbred replacement heifers (Figure 1A) were consigned from 16 Maritime beef producers (Figure 1B) and housed at the Maritime Beef Testing Society in Nappan, Nova Scotia. Heifers were evaluated for productive performance (individual feed intake, growth and body composition) for 124 days, while consuming a grass-silage based diet. A CBC assessment was implemented at the start and the end of the performance test, while analysis of blood plasma metabolites was performed every 28 days throughout the performance evaluation. Additionally, heifers were vaccinated with ovalbumin (OVA), which was followed by a booster shot 14 days later. This regime of immunization enabled this study to assess the specific primary and secondary immune responses of immunoglobulin (Ig) IgG1 and IgM to OVA.

Heifers were classified as either efficient (54 heifers; low-RFI = -1.02 kg/day less dry matter consumed than expected) or inefficient (53 heifers; high-RFI = +1.06 kg/day more dry matter consumed than expected) and the means of these feed efficiency groups were compared. All reported results were statistically different.

Beef cows in feedlot

Figure 1A: Heifers evaluated during the experiment.

Map of Maritime provinces with stars showing locations of calves

Figure 1B: Maritime beef producers, across the three provinces who had 107 heifer calves tested in this study.

Complete Blood Cell Count

The CBC is an assessment routinely used to evaluate health status in cattle. The analysis includes red blood cell count, white blood cell counts, platelets and concentrations of proteins such as hemoglobin, which are related to energetic fluctuations associated with variation in feed intake. The results indicated that efficient heifers had a lower mean cell hemoglobin (Figure 2A), but no difference in red blood cell count. This suggests that efficient heifers may have lower oxygen carrying capacity, which complements the previous findings of our labii, where a lower blood plasma CO2 concentration was observed in inefficient beef heifers (Gonano et al 2014), since these two blood gasses are closely related to metabolic rate.

Series of three bar graphs with a blue bar on left showing efficient heifers and red bar on right showing inefficient heifers. The graph on the left is mean cell hemoglobin. The graph in the middle is for neutrophil and the graph on the right is for lymphocyte values.

Figure 2: (A) Mean cell hemoglobin, (B) neutrophil and (C) lymphocyte values of the efficient and inefficient heifers at the start of the trial with 287 ± 27.6 days of age.

The CBC analyses also identified differences in white blood cells (Figures 2B and 2C), in relation to feed efficiency, suggesting differences in cellular immune function. The lower oxygen requirements in the feed efficient heifers may offer a greater amount of oxygen to be metabolized by the lymphocytes, which could lead to the larger number of lymphocytes observed. Higher abundance of segmented neutrophils in inefficient heifers could be related to susceptibility to stress. Increased segmented neutrophil abundance in mildly stressed cattle is related to a cortisol surge, causing the release of neutrophils from blood vessel walls.

Immune System Challenge Using Ovalbumin

Maintenance of the immune system requires a considerable amount of energy that may reduce energy allocations toward growth and reproduction, among other metabolic sinks. It has been observed that response to an infection can reduce protein accretion in muscle, resulting in decreased productive performanceiii. In cattle, Ig M and G1 are primary responders of the humoral immune system required to detect pathogens. Efficient heifers displayed 43% greater secondary IgM response to OVA, indicating a superior immune function, which may relate to their improved feed efficiency. Despite the lack of differences in IgG1, the results of IgM are encouraging, given the well-known association of IgM with first detection of a novel antigen.

Blood Plasma Metabolite Profile

Series of three bar graphs with a blue bar on left showing efficient heifers and red bar on right showing inefficient heifers. The graph on the left is blood plasma metabolite concentrations for phosphorus. The graph in the middle is for potassium and the graph on the right is for alkaline phosphatase.

Figure 3: Blood plasma metabolite concentrations in relation to feed efficiency, (A): phosphorus; (B): potassium and (C): alkaline phosphatase.

Blood metabolites may provide information about the underlying biochemistry associated with individual variation in feed efficiency. Blood plasma measurements, such as mineral ions, proteins, compounds, enzymes and hormones, can be related to processes that affect energy use. Efficient heifers had greater concentrations of phosphorus (Figure 3A). Plasma phosphorus contributes to the production of muscle storage molecules, including creatine phosphate and ATP, which serve as readily available energy sources. Efficient heifers also had greater potassium levels (Figure 3B) that may be related to a lower rate of protein turnover, leading to more efficient growth in efficient heifers; however, this explanation still requires further research. Inefficient heifers had higher levels of alkaline phosphatase (ALP) (Figure 3C), which may relate to the increased transport of phosphates across cellular membranes, supporting a greater demand for background metabolic processes, rather than for growth. Efficient heifers also had lower levels of T3 (triiodothyronine), in comparison to the inefficient heifers (1.8 vs. 1.9 mmol/L). This hormone, produced by the thyroid gland, is key in the regulation of metabolism.

Implications

  • Parameters of the CBC analysis, including hemoglobin concentration and white blood cell profile, may constitute simple assessments of the intricate biology underlying feed efficiency.
  • Heifers with improved feed efficiency may have superior humoral immune capacity, based on the specific response of immunoglobulin M to a novel antigen (ovalbumin).
  • Blood plasma metabolites such as T3, ALP, potassium and phosphorus may contribute to the indirect assessment of feed efficiency, as part of routine beef cattle testing protocol.

Acknowledgements

We thank the Beef Cattle Research Council, Agriculture and Agri-Food Canada, Nova Scotia Department of Agriculture, New Brunswick Cattle Producers, Nova Scotia Cattle Producers, Prince Edward Island Cattle Producers and the Maritime Beef Testing Society for their support.

References

i Pitchford WS (2004) Genetic improvement of feed efficiency of beef cattle: what lessons can be learnt from other species? Australian Journal of Experimental Agriculture 44, 371-382.

ii Gonano CV, Montanholi YR, Schenkel FS, Smith BA, Cant JP, and Miller SP (2014) The relationship between feed efficiency and the circadian profile of blood plasma analytes measured in beef heifers at different physiological stages. Animal 10, 1-15.

iii Klassing KC (1988) Nutritional aspects of leukocytic cytokines. Journal of Nutrition 118, 1436-1446.


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