Blood traits as net feed conversion efficiency indicators in Angus cattle

Why the question mattered#

Breeding programmes had traditionally focused on lifting output traits such as growth and carcase quality, but the paper notes that rising input costs meant breeders also needed ways to select animals that use feed more efficiently without constantly increasing yield. Net feed conversion efficiency, described in the paper as feed intake measured against the amount expected for an animal's maintenance and growth, offered a way to identify cattle that eat less than expected for the same output. Cattle with a high value for this trait, by the paper's own description, eat less than their lower-ranked counterparts of similar size and growth rate.

The difficulty is that measuring the trait directly means putting every animal through an individual intake test lasting many weeks, which is costly and logistically demanding at scale across a commercial breeding operation. Richardson and colleagues set out to see whether simpler measures, taken from a blood sample or a faecal sample, might track closely enough with net feed conversion efficiency to work as early indicators, potentially easing reliance on full testing down the track. This is why the paper frames its own results carefully, as a first step toward a screening approach rather than a finished tool ready for immediate use on farm by commercial breeders.

How the Trangie cattle were tested#

The animals came from the second and third rounds of feed efficiency testing run at Trangie Agricultural Research Centre, with fuller protocol details given in a companion paper by Arthur and colleagues. The second round involved Angus, Hereford and Shorthorn heifers sourced from industry herds, numbering 193 head. The third round drew on heifers and bulls bred at Trangie itself, with 194 heifers and 188 bulls recorded. Every animal ate a pelleted ration of 70% lucerne hay and 30% wheat, delivered through an automated system and tracked across a 120-day period, and cattle were then ranked according to the gap between expected and actual intake over that period of testing.

From each round, researchers sampled the ten animals ranked highest and the ten ranked lowest for net feed conversion efficiency, taking blood and faeces once testing finished. Animals in the third round were also bled a month earlier, which let the team check whether measurements stayed consistent over time within the same animal. Blood was analysed for twelve haematological parameters plus insulin-like growth factor-1 and insulin-like growth factor-2, while alkane concentrations in feed and faeces were used to work out dry-matter digestibility for each animal, following methods described by Dove (1992), a published reference the authors drew on directly for the laboratory analysis.

Net feed conversion efficiency indicators found in blood#

Many of the measured blood parameters, particularly white blood cells, lymphocytes, neutrophils, monocytes and eosinophils, proved highly variable and showed no clear pattern linked to efficiency ranking. Among the steadier constituents, however, several differences stood out clearly in the analysis. Total plasma protein differed between the two groups (P less than 0.01). The ratio of haemoglobin to red blood cells, labelled MCH in the paper, showed a strong difference (P less than 0.01) and was also highly repeatable on retesting a month later. The ratio of haematocrit to red blood cells, MCV, was significant at a lower threshold (P less than 0.05), and the plain count of red blood cells came close to significance (P less than 0.10) without quite reaching it in the analysis.

A separate plasma protein measurement, taken from a different blood tube and analysed by a different laboratory, also differed between the two groups once testing ended, at 68.3 plus or minus 0.6 versus 70.1 plus or minus 0.6 grams per litre (P less than 0.05), with the less efficient animals again showing the higher value. The ratio of albumin to this plasma protein measure, however, did not differ between groups, which the authors read as evidence against one group carrying a heavier load of globulins, and therefore against greater immune exposure in either group of cattle tested.

Repeatability over time and what it implies#

Because the third-round animals were bled both a month before and right at the end of testing, the researchers could check whether the traits separating efficient from inefficient cattle held steady within an individual animal over that interval. The MCH ratio proved highly repeatable, with a correlation above 0.9 across the two sampling occasions, a strong result for a biological measurement of this kind taken a month apart. The MCV ratio, total plasma protein and red blood cell counts were moderately repeatable, with correlations above 0.5 between the two time points of sampling.

This distinction matters because a trait that swings unpredictably within an animal over a few weeks would be a weak candidate for early screening, even where it differs between efficiency groups at a single point in time. The paper also flags that haemoglobin in both sexes, and haematocrit and lymphocytes in bulls specifically, were moderately repeatable yet showed no link to efficiency differences, a reminder that repeatability on its own does not make a measurement useful for this particular screening purpose.

Digestibility differences between efficient and inefficient cattle#

Alongside the blood work, the study used alkane markers in feed and faeces to estimate dry-matter digestibility for each animal, drawing on techniques published by Dove (1992). Because gathering enough faecal samples from each animal is itself costly and time-consuming, the researchers first checked how many samples were truly needed for a reliable estimate of average alkane content. Using twelve animals from the second round with four samples apiece, they found that averages built from only one or two samples correlated poorly, below 0.5, with the four-sample average, while three samples produced a correlation of 0.67, judged acceptable for use in the main analysis going forward.

Overall dry-matter digestibility came out around 66% in the second round and around 69% in the third round of testing. Across both rounds, cattle with higher net feed conversion efficiency showed a tendency toward better digestion than lower-ranked cattle, at 68.1 plus or minus 0.5% versus 67.1 plus or minus 0.5% (P less than 0.10). The paper argues this one percentage point gap is not trivial, citing calculations from MAFF (1984) suggesting a one percentage point lift in digestibility could cut daily feed needs by 2.3% for growing cattle of a weight and gain typical of the trial animals used here. The overall gap in average feed intake between the two groups during the sampling window was about 16%, so the digestibility effect was estimated to explain roughly 14% of that observed gap in intake between groups.

Why growth hormone measures did not separate the groups#

Insulin-like growth factor-1 has previously been linked to growth, body size, feed conversion and carcase traits, as cited from Davis and Bishop (1994), which made it a reasonable candidate for testing in this particular context. Yet in the second round, neither IGF-1 (276 versus 249 picograms per millilitre) nor IGF-2 (174 versus 180 picograms per millilitre) differed significantly between the two efficiency groups, with P greater than 0.05 recorded in each case tested.

The authors offer a specific explanation for this negative result. Net feed conversion efficiency is worked out after removing the statistical influence of liveweight and growth rate, which may be why it shows a weaker link to IGF-1 than other feed conversion measures that leave those influences in place. They also note that circulating IGFs travel bound to binding proteins that limit their passage through capillaries and restrict access to cell receptors, following the mechanism described by Zapf and Froesch (1986) in earlier published work. A simple total plasma reading, in other words, might still miss a genuine role for growth hormone pathways even where no statistical difference shows up in the data collected.

What this means for breeding programs#

The paper is explicit that every result reported here comes from sampling done once each test had already finished, so the blood and digestibility differences could be a by-product of how animals performed during testing rather than traits that exist independently beforehand. The authors state plainly that more research is needed to find differences present before testing begins, which would be genuinely predictive and could justify a cheaper screening step ahead of a full protocol such as the one described in the related work on residual feed intake in grazing and restricted-fed Angus cows.

Given that the comparison groups in this study were drawn from only the ten highest and ten lowest ranked animals in each round, and that the significant results are associations rather than demonstrated causes, we would treat the plasma protein and red cell ratio findings as a promising lead rather than a ready-made screening tool for commercial use on farm. The paper's own caution about end-of-test sampling supports that reading. Related work on testing more cattle for residual feed intake and the broader ASAP Conference Proceedings collection sit alongside this paper within the same long-running Trangie research program into feed efficiency in beef cattle.

Sources and further reading#

• Australian Association of Animal Sciences: successor to ASAP
• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body
• BREEDPLAN genetic evaluation: genetic evaluation system for Australian beef cattle

Questions#

— The Livestock Library team