04/10/2025
Lots of valuable information in this post and also comment section.
💬 “I know I often go on about capeweed 🙄 … but a recent client’s senior horse really got me thinking.”
👉 This older horse has started coughing and struggling with exercise intolerance again — almost exactly the same time as last year.
👉 It made me wonder: could capeweed, now flowering and filling paddocks, be making things worse?
Most people think the risk is just about sugars (capeweed can hit over 30% WSC!) and laminitis.
👉 But that’s only half the story…
🌅 Morning risk – nitrates/nitrites
👉 Capeweed and flatweed accumulate nitrates overnight.
👉 In the hindgut, these convert to nitrites, which oxidise haemoglobin into methemoglobin — a form that can’t carry oxygen.
👉 Result: 🩸 less oxygen in the blood, 😮💨 breathing struggles, 💔 poor circulation, 🐴 biome stress.
☀️ Afternoon risk – sugars (WSC)
👉 Photosynthesis drives sugars up later in the day.
👉 Result: 📈 insulin spikes, 🐎 laminitis trigger.
🌼 Capeweed = triple hit
1️⃣ Nitrite burden in the morning (oxygen transport + biome disruption)
2️⃣ Sugar burden in the afternoon (insulin & laminitis risk)
3️⃣ Pollen allergen when flowering (airway inflammation, asthma flares)
💪 Muscle & Nerve Stress (Stringhalt link)
👉 Capeweed & Flatweed are linked to Australian Stringhalt.
👉 WA pastures are often low in magnesium (Mg) and sodium (Na) — both vital for nerve conduction and muscle relaxation.
👉 Low Mg/Na + nitrite stress = twitchiness, abnormal gait, poor stamina, and stringhalt‑like signs.
👉 Add this to asthma and circulation issues → seniors especially struggle with exercise intolerance and recovery.
⚡ The bigger picture:
👉 Capeweed and flatweed don’t just stress the lungs and hooves — they also stress the muscles and nerves.
👉 Seniors, with decades of exposure and reduced gut resilience, are hit hardest.
✨ Take‑home: With capeweed, it’s not just about sugars.
👉 Nitrates and nitrites from weeds like capeweed and flatweed can affect your horse’s blood oxygen, circulation, muscles, and gut biome — especially in seniors or horses with asthma and laminitis risk.
📚 References
Asmala, T. et al. (2019). Nitrate and nitrite in forage crops and risk of toxicity in herbivores. Grass Forage Sci, 74(4):511–523.
Arai, N. et al. (2011). Methemoglobinemia due to nitrite ingestion in horses. J Vet Med Sci, 73(10):1389–1391.
NRC (2007). Nutrient Requirements of Horses. National Academies Press.
Couëtil, L.L. et al. (2020). Equine Asthma: Current understanding and future directions. Front Vet Sci, 7:450.
Grace, N.D. et al. (2010). Mineral composition of pastures and implications for grazing animals. NZ Vet J, 58(3):118–123.
Waldridge, B. (2010). Nitrate and Nitrite Toxicity in Horses. Kentucky Equine Research.
Robertson, S.A. & Muir, W.W. (2013). Equine Anaesthesia. Saunders.
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