09/12/2025
As we head into summer, the conversation around grass staggers shifts a little. Spring flush may be behind us, but high-endophyte perennial ryegrass starts to play a much bigger role. Many NZ farms rely on these tougher, heat-tolerant grasses to carry the herd through dry spells, but with them comes a different kind of risk.
High endophyte grasses produce compounds that help the plant survive insects, heat and drought. Great for the pasture… not always great for the cow. These endophyte toxins can stress the animal’s nervous system, making them more reactive, unsettled, and sensitive to handling. At the same time, these summer ryegrass pastures can still be low in magnesium, especially when growth slows or conditions turn dry.
It’s this combination of lower magnesium availability and higher endophyte challenge that sets the stage for grass staggers in the warmer months. The signs can look slightly different too. Cows may be more twitchy, flighty, or anxious before you see the classic uncoordinated gait or muscle tremors that we associate with grass staggers.
Hot weather, dry spells, and sudden shifts onto harder, stalkier ryegrass can all tip the balance. Older cows and high producers remain the most vulnerable, especially if they’re already carrying a bit of stress.
Prevention heading into summer is about staying ahead of these pasture changes:
• Keep magnesium supplementation steady, even though calving is behind you.
• Be cautious with sudden moves onto high endophyte paddocks, especially in heat or after a dry spell.
• Offer hay or silage to help the rumen handle tougher summer feed.
• Maintain good observation — behaviour changes are often the first red flag.
• Consider additional support such as GS Nosode to help reduce the impact of endophyte stress.
Grass staggers in summer isn’t just a mineral story - it’s a pasture story, an endophyte story, and a stress story. But with good management and early anticipation, it’s a risk we can stay well ahead of.