Protein digestion in cattle shows spear grass hay falls well short

Why the question mattered for tropical pastures#

Cattle grazing mature tropical grasses often perform poorly compared with those on temperate pastures, and the usual fix is a supplement containing protein protected from microbial breakdown in the rumen. Hunter and Siebert set out to test why some tropical forages seem to leave cattle short of protein even though they appear to carry adequate crude protein on paper.

The authors point out that in sheep and cattle fed temperate forages, the proportion of crude protein digested in the intestines typically runs from 0.65 to 0.75. Earlier work by the same authors and by Kennedy had suggested that cattle fed mature tropical forages digest far less of the protein that reaches their intestines, which would leave a poor balance between the protein and energy an animal actually absorbs. If this pattern was common across tropical forages, it would help explain the weak growth response seen in grazing cattle and the gains achieved when rumen-protected protein supplements are added to their diet.

To test this, the study compared two contrasting tropical hays: spear grass (Heteropogon contortus), a highly fibrous grass that resists digestion in the rumen, and Pangola grass (Digitaria decumbens), which breaks down more readily there. The aim was to measure, directly in the live animal, how much crude protein flowed into the small intestine from each diet and how much of that protein the animal actually digested and presumably absorbed.

How protein digestion in cattle was measured#

The work, described in full in the ASAP Conference Proceedings collection, was carried out as two experiments using steers fitted with fistulas, surgical openings that allow sampling, in the abomasum and rumen. Five Droughtmaster steers were fed long-chopped spear grass hay to appetite, while four Hereford steers were fed long-chopped Pangola grass hay at close to their full intake. Feed was offered in equal portions at regular intervals through automatic feeding devices, so intake stayed steady across the day.

Digesta flow through the abomasum, the compartment just before the small intestine, was tracked using a continuously infused marker solution together with lignin, a fibre component that passes through largely unchanged. The chromium marker was infused for at least three days before and during sampling. Flow calculations for the spear grass diet followed the method of Weston and Hogan, while the Pangola grass data used the method described by Faichney, reflecting the two separate experiments behind this combined report.

Alongside the flow measurements, the authors characterised the feeds themselves. Amino acid composition was determined by hydrolysing samples with hydrochloric acid and running them through an amino acid analyser. They also measured how much nitrogen was soluble when treated with the digestive enzyme pepsin, and how much nitrogen was tied up in the cell wall material of each grass, using standard detergent fibre analysis. These chemical measurements were meant to explain any difference in digestibility found between the two grasses, not just describe it.

What the comparison of spear grass and Pangola grass found#

Spear grass carried less nitrogen and noticeably more lignin than Pangola grass, by 33 per cent and 26 per cent respectively, a combination that points to both lower protein content and tougher, more fibrous structure. Steers eating spear grass, despite being fed to appetite, took in only about two-thirds as much digestible organic matter, a measure of digestible energy, as the Pangola grass steers, even though the Pangola group was fed at around 85 per cent of appetite.

The protein findings were stark. Crude protein flowing into the intestines averaged 225 grams per day on spear grass and 443 grams per day on Pangola grass. Of that, the steers on spear grass digested only 30 per cent, while those on Pangola grass digested 68 per cent. Nitrogen balance mirrored this: spear grass steers excreted more nitrogen in faeces than they ate in feed and lost 18 grams of nitrogen from the body per day, while Pangola grass steers held a slight positive nitrogen balance.

The ratio of digestible protein to digestible energy, expressed as DCPi:DOMI (digestible crude protein in the intestines relative to digestible organic matter intake), came out at 0.05:1 for spear grass and 0.12:1 for Pangola grass. That difference means spear grass delivers far less protein for the energy an animal gets from it, a mismatch that would be expected to limit growth even when energy intake looks adequate on paper.

Why spear grass protein resists digestion#

The authors looked for a chemical explanation for why spear grass nitrogen was so poorly digested. Only 25 per cent of total nitrogen in spear grass was in the form of amino acids, compared with 75 per cent in Pangola grass. Treating the feeds with acid-pepsin, mimicking stomach digestion, removed just 37 per cent of spear grass nitrogen but 56 per cent of Pangola grass nitrogen. Both feeds, however, had a similar proportion of total nitrogen, 42 per cent, bound up in cell wall material.

That last finding matters because it rules out cell wall binding as the whole story. Since both grasses had a similar share of nitrogen tied to the cell wall, at least part of the indigestible nitrogen fraction in spear grass must sit outside the cell wall yet still resist digestion by enzymes and by rumen microbes. The authors state plainly that the precise chemical nature of this resistant nitrogen fraction remains unknown and deserves further study.

Discussing a related finding from Kennedy, the study notes that a diet based mainly on Panicum maximum pasture hay gave a similarly low DCPi:DOMI ratio of 0.05:1 in steers, compared with a notably higher ratio for lucerne (Medicago sativa). The authors draw a tentative link between forage growth habit and protein digestibility, pointing out that spear grass and Panicum maximum share an upright, clumping growth form, while Pangola grass spreads low across the ground, and suggesting this upright habit may be associated with a resistant nitrogen fraction. This comparison echoes themes covered in a related record on predicting crude protein supply to grazing sheep.

How confident can we be in these figures#

The study measured the digestibility of protein reaching the intestines without separating out the animal's own contribution to faecal nitrogen, meaning faecal nitrogen includes both undigested dietary protein and nitrogen from the animal's own gut lining and secretions. Because spear grass supplied less nitrogen overall, this endogenous component would make up a larger share of the faecal total on that diet, which could exaggerate the apparent difference between the two grasses.

The authors address this directly and argue it is unlikely to be the sole explanation, pointing out that the pasture hay used by Kennedy had a higher nitrogen intake than either the spear grass or Pangola grass diets in this study, yet still produced a similarly low DCPi:DOMI ratio. That comparison across independent studies lends some weight to the idea that the pattern is real rather than an artefact of measurement.

We would treat the core finding as a solid demonstration that these two particular tropical hays differ sharply in intestinal protein digestibility, but the sample sizes were small, five and four steers respectively, and the explanation for the resistant nitrogen in spear grass remains unresolved rather than proven. Readers should view this as a well-documented case study rather than a settled rule covering all erect tropical grasses.

Connecting the findings to a practical feeding trial#

The paper draws out the practical stakes by citing the ARC's guidance that cattle heavier than 100 kg fed roughage diets supplying 0.13 grams of DCPi per gram of DOMI can meet their protein needs from the roughage alone, without extra protein supplementation. Pangola grass, at 0.12:1, sits close to that threshold, suggesting steers on this hay would gain little from added protected protein. Spear grass, at 0.05:1, falls well short, implying a clear protein deficiency even for maintenance.

Supporting evidence comes from a trial by Lindsay and Loxton, cited in the discussion, in which steers fed spear grass and supplemented with 500 grams per day of a protein supplement based mainly on formaldehyde-treated cottonseed meal gained 109 grams per day, compared with a loss of 320 grams per day in steers given only rumen-degradable nitrogen and sulphur. The authors note that some of this difference could reflect the extra energy in the cottonseed meal supplement, since it is roughly 40 per cent crude protein, but argue the energy supplied was not enough on its own to account for the full weight gain difference, so improved protein supply was likely the main driver.

Taken together, the two pieces of evidence point the same way: on fibrous, low-nitrogen tropical grasses like spear grass, protein protected from rumen breakdown can lift live weight gain substantially, while on more digestible grasses like Pangola, the protein supply from the plant itself may already be adequate relative to its energy content.

What this means for protein supplementation decisions#

The authors conclude that tropical pasture hays vary widely in how much digestible protein they deliver to the intestines relative to their energy value, and that this variation has direct consequences for whether a protein supplement will pay off. Their suggestion is that effective and economical supplementation programs need to be based on actual knowledge of how a particular pasture's protein is digested, rather than assuming all tropical roughages behave alike.

This matters because crude protein content alone, as measured by standard feed analysis, does not tell the full story. Two hays with broadly similar crude protein levels could still differ enormously in how much of that protein an animal can actually use, depending on how resistant the nitrogen fraction is to digestion in the gut. The chemical and physical tests used here, including pepsin solubility and amino acid analysis, offer one way to flag forages likely to be protein deficient in practice even when their crude protein figure looks reasonable.

For producers managing cattle on erect, tussocky tropical grasses resembling spear grass, the findings support targeted use of rumen-protected protein supplements during periods when such hays or pastures dominate the diet. For those on more prostrate, readily digested species like Pangola grass, the case for additional protein is weaker on the evidence presented here. This paper sits within the wider body of work held in the Australian Society of Animal Production community, alongside broader resources available through the Livestock Library research index.

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
• Integrity Systems (NLIS and LPA): which runs NLIS, LPA and the National Vendor Declaration

Questions#

— The Livestock Library team