Oxalate is found in weeds such as fat hen, yarr, oxalis and sorrel, and also in the leaves of Fodder beet.
Oxalate binds to calcium in the blood causing hypocalcaemia (milk fever), but relatively large quantities need to be consumed for this to happen. We typically see this syndrome in dairy cows put onto a break of weed-infested young grass after milking, resulting in several down or even dead cows. We have also seen this in dry cows who break out on Fodder beet and consume large amounts of leaf. Most cows respond well to treatment with intravenous calcium if found in time. Long term exposure to lower levels of oxalate causes kidney failure, due to the oxalate crystals damaging the kidneys, and/or damage to the blood vessels in the lungs and rumen. This syndrome is more commonly seen in sheep or dry stock and unfortunately doesn't usually respond well to treatment.
Dusting the young grass with lime flour at 150g/head prior to grazing may help reduce the incidence of oxalate poisoning by binding to the oxalate before it gets into the blood. Mixing breaks of infested pasture with clean pasture is also sensible, but topping and wilting the plant may NOT necessarily reduce its toxicity.
The concentration of oxalates in the plant varies with fertilizer applications, moisture content of the soil and the maturity of the plant. Unfortunately this varies between species so rather than try to predict whether a paddock is safe, it is best to assume that it is not.
Weeds can also accumulate nitrate and have the potential to cause nitrate poisoning. It is important to differentiate between this and oxalate poisoning as both can cause down cows, but the treatment is quite different; it pays to seek veterinary help if you encounter down cows after they have been exposed to these weeds.