Investigating the risk of oxalate poisoning for cows grazing irrigated buffel grass

Buffel grass is a valued pasture species for livestock in Central Australia. Typically, it occurs with native grasses and is capable of producing large amounts of palatable, nutritious feed. Buffel grass also does well under irrigation, providing a secure source of feed in a region that is known for low and unreliable rainfall. While growing irrigated fodder for grazing as pasture isn’t common in Central Australia, the managers at Territory Grape Farm (TGF), Janet and Roy Chisholm, have access to a good supply of water and the infrastructure necessary to grow approximately 50 hectares of buffel grass under pivots.

Janet says, “We wanted to create a body of feed for our cows and calves regardless of rainfall. It was pretty clear that buffel grass would do well with irrigation but we were also aware that it can cause oxalate poisoning, so we wanted to find out more.”

Buffel grass can have very high concentrations of soluble oxalates, particularly when it is young and actively growing. Under certain conditions, such as when growing as a monoculture or at the end of extended dry periods, high oxalate concentrations may lead to health problems for livestock if the soluble oxalates are absorbed into the blood stream and bind to calcium ions. For particular groups of cattle, this can cause either acute hypocalcaemia or the formation of calcium oxalate crystals.

Calcium oxalate crystals that form in the kidney tubes can interfere with kidney function and cause urinary obstruction. Calcium oxalate crystals can also damage blood vessels, as well as organs including the liver and the rumen wall. Although the occurrence of oxalate poisoning in cattle is probably very low for well-managed stock, groups of cattle that are most susceptible include hungry animals, travelling stock, those not accustomed to high oxalate pastures, first-calf heifers and young male cattle with periodic low water consumption.

From July 2019 to June 2020, researchers from the Northern Territory Department of Industry, Tourism and Trade, in conjunction with the managers of Territory Grape Farm, undertook a study to investigate soluble oxalate levels in buffel grass under irrigation. Located approximately 180 km north of Alice Springs, Territory Grape Farm has uniform red, sandy soils. Rain can occur at any time of the year but mostly in summer (warm wet summers and cool, dry winters). The area doesn’t typically get frosts in winter and it is usually warm enough for some buffel grass growth to occur all year round if there is sufficient soil moisture.

What did we do?

Pasture samples were collected from irrigated buffel grass that was either grazed or ungrazed. We also sampled grazed buffel grass at a non-irrigated site (rain-fed). Pasture samples were collected throughout the year and sent to laboratories specialising in plant nutritional analysis.

Because monocultures of irrigated buffel aren’t common in Central Australia, we also included pasture samples from two sites (not irrigated) on Old Man Plains Research Station (OMP), located 20 km south of Alice Springs. These sites have slightly more fertile soils than at TGF and support a mixture of grasses, including buffel grass and native grasses, such as oatgrass (Enneapogon avenaceus) and button grass (Dactyloctenium radulans).

At the Territory Grape Farm, 120 Santa Gertrudis crossbred and Droughtmaster cows, and their progeny (65 calves), were put on the buffel pasture in early April, three months prior to the commencement of irrigation in July 2019. Blood samples were collected from 30 individual cows in April 2019. Twenty-four cows were resampled in June 2020 after 12 months grazing the irrigated buffel grass.

Comparing irrigated and rain-fed pastures

Irrigated buffel grass was grown under two 25-hectare centre pivots. The pivots applied water on alternate days to deliver 50 mm of water per week over the study period.

During the study, Territory Grape Farm received 215.6mm of rain, a little lower than the median of 297.8 mm. Most of the rain fell in an event at the end of January, extending into early February, with follow-up rain in early March. This meant that rain‑fed buffel grass on TGF was actively growing from February to May 2020. The sites on OMP also received rain (93 mm) in late January and early February resulting in a pasture growth event.

Buffel grass grows well in warm conditions and daily maximum temperatures were suitable for the irrigated buffel grass to grow for pretty much the entire study period. However, it is likely that growth would have been slower during July and August 2019, and again from May to August 2020, when temperatures were cooler.

Oxalate levels linked to irrigation, growth phase and moisture content

Soluble oxalate levels in the irrigated pasture were consistently above the recommended <2% of dietary dry matter (DM) for the measured periods. In contrast, in the non-irrigated buffel, on both TGF and OMP, soluble oxalate levels only exceeded the recommended threshold for short periods after the rain-fed growth event in late January and early February 2020 (Figure 1).

Figure 1.  Soluble oxalate percentage in buffel grass over the study period separated by grazing and watering treatment and sampling site (Note: The broken lines only connect data collected prior to irrigation or rain initiated pasture growth to data post pasture growth and do not represent the change or trends in soluble oxalate levels during those periods)

 

Oxalate levels can vary depending on the growth stage of a grass plant. Soluble oxalate levels were especially high during the early growth phase, when the buffel had lots of fresh, green leaves. This occurred in both the irrigated pastures and the rain-fed pastures. Under irrigation and consistent grazing, buffel grass was kept in this early growth phase. Without grazing, the irrigated buffel reached maturity. Grazing pressure on the rain-fed buffel at OMP was much lower and these plants also progressed to maturity.

When rain occurred, all treatments showed a spike in soluble oxalates to above 4% DM (more than 2% above the recommended threshold). When it rains, nitrogen from the atmosphere is dissolved in the raindrops and is then readily absorbed by plants. An increase in nitrogen can cause changes in oxalate accumulation in certain plants (Libert and Franceschi 1987). This could explain the spike in soluble oxalates, even in irrigated pasture. The soluble oxalate concentration only remained at this peak for a short period before the buffel grass matured and soluble oxalate concentration declined.

There was a strong correlation between moisture content and soluble oxalate concentration. As the moisture content of the grass increased, so too did soluble oxalate levels (Figure 2). In terms of management, the results suggest that the risky level of soluble oxalate may drop significantly once the moisture concentration of the buffel grass falls below 70%.

Figure 2. Moisture vs soluble oxalate percentage of buffel grass

Non-oxalate plants can help lower the dietary intake

The OMP site is more typical of an extensive rangeland pasture where cattle have access to a variety of grass species. After the January and February rainfall, soluble oxalate levels in the buffel grass peaked with almost twice the recommended threshold percentage of dietary DM, but levels dropped quickly as the plants matured. The two native grasses tested did not contain oxalates at any stage so, if buffel grass contained a high level of soluble oxalate, cattle grazing a mixed diet with those grasses could have a better chance of a dietary intake below the recommended threshold level for reduced risk of oxalate poisoning. It is also possible that oxalate levels only increased above the recommended daily limit for a short period and that animals didn’t consume enough over that time to be badly affected.

How did the high levels of soluble oxalate in the irrigated buffel grass affect the cattle?

Blood samples were collected twice from breeder cows at TGF: 3 months prior to irrigation and again at the end of the study (photo 1). The samples were analysed for calcium, albumin and phosphorus because the concentration of these can all be affected by a high oxalate diet. Urea and creatinine levels were also measured as an indicator of kidney health.

Photo 1: Bryan Gill (DITT) taking pre-irrigation blood samples at the Territory Grape Farm (11 April 2019).

Despite soluble oxalate concentration being high, all the blood parameters tested remained in the normal range. Calcium levels were slightly lower at the end of the study, but well within the normal range for good health.

At the end of the study, albumin levels were also slightly lower than the initial levels but this could have been due to possible dehydrated of the cattle when the initial samples were taken. Slightly elevated albumin levels are not uncommon in hot regions and may be a symptom of dehydration.

Phosphorus levels were slightly higher in 2019 compared with 2020, but were still within the normal range.

Urea and creatinine, which are good indicators of kidney health, were significantly different between the two sampling periods but were also well within the normal range. Based on the elevated albumin, the differences between urea and creatinine in 2019 and 2020 may also be explained by slight dehydration in the mob at the first bleed (Shilton pers. comm. 2020).

Management options to reduce oxalate poisoning risk

  • While buffel grass growth is in the early growth phase, either provide cattle (especially unadapted animals) with a supplementary low oxalate feed source or allow access to unirrigated pasture.
  • Sow an additional low oxalate pasture species into the buffel grass pasture to dilute the dietary oxalate levels, for example, a legume.
  • Incorporate a rotational grazing system into the irrigated buffel grass grazing system to prevent grazing of pasture in the early growth phase. By allowing buffel plants to establish and mature, such a grazing strategy might also facilitate greater pasture production, as individual tussocks are likely to develop better root systems and become more robust.

Conclusion

Soluble oxalate levels in the irrigated, constantly grazed buffel grass remained higher than the recommended <2% of dietary DM. Although the cattle in the study were exposed to extended periods of potentially toxic soluble oxalate levels, their blood parameters did not indicate evidence of oxalate poisoning. Cattle rumen flora have the ability to adapt to high oxalate levels. In this instance, the cows had been exposed to a diet high in buffel grass before the study. It is likely that their rumen flora adapted to break down oxalates which reduced potential ill effects of high oxalate levels. That aside, the study did identify several key management options that would help minimise the likelihood of oxalate poisoning in ‘at risk’ cattle.

As Janet says, there are still many questions to answer, such as, “It was really interesting that the highest spike in oxalates was after the summer rain. How much does the nitrogen in the rain contribute to that spike and would that be something to consider if we decided to use nitrogen to increase grass growth under irrigation?”

Acknowledgements

Special thanks go to:

  • Janet Chisholm of Territory Grape Farm
  • Roy Chisholm and TGF staff for mustering cattle
  • Bryan Gill (Department of Industry, Tourism and Trade Manager OMP) for collecting blood samples
  • Cathy Shilton (Department of Industry, Tourism and Trade Principle Veterinary Pathologist) for advice and interpretation of the blood results
  • Mark Hearnden (Department of Industry, Tourism and Trade Principle Scientist, Biometry) for advice on statistical analysis
  • Kirsten Skinner (Department of Industry, Tourism and Trade Technical Officer) for assistance in bleeding.

References

Eccles, J. (2010). Obstructive Urolithiasis (Bladder Stones) in Cattle. Agnote: K57. Northern Territory Government, https://industry.nt.gov.au/__data/assets/pdf_file/0011/233579/844.pdf

Libert, B. and Franceschi, V.R. (1987). Oxalate in Crop Plants. Journal of Agricultural and Food Chemistry, 35, 926‑938.

This article has been written by Chris Materne and Alison Kain, Pastoral Production Officers, Department of Industry, Tourism and Trade.