Rusty Ryegrass: When green goes orange

By Charlotte Westwood, Veterinary Nutritionist at PGG Wrightson Seeds

Given the changeable weather in late summer and early autumn months, fluctuating between too hot, too dry, too wet too humid and alike, it’s not surprising the ryegrasses (and tall fescues) in our pastures have in some cases turned an odd shade of orange.

Rust - otherwise known as Crown rust (Puccinia coronata) or it’s ‘cousin’ Stem rust (Puccinia graminis).

Crown rust is found throughout New Zealand and is the most common type of rust that occurs. Stem rust tends to be a South Island issue, most often seen in the autumn.

As a fungus, a surge in warm, moist weather sets off the right conditions for rust to grow. Nitrogen deficiency in grass also contributes to a greater risk of rust.

Should we be concerned about grazing animals on rust-affected pastures? As far as animal health issues go, there is no specific animal health issues to worry about if stock graze rusty pasture, as far as consuming the Puccinia fungi.

With that said, rusty pasture doesn’t grow as well compared to green, healthy grass. Stock also have a low preference for grazing rust-affected ryegrass and therefore, dry matter intake and pasture utilisation can be reduced. An accumulation of facial eczema (Pithomyces chartarum) spores increases in warmer regions of New Zealand when rusty, dead or dying pasture material builds up in the base of the pasture sward.

In the short term, when you’re inspecting paddocks if you can only see minor areas of rust (i.e. orange discolouration) then keep on grazing. If taller paddocks are fairly orange, consider baling those paddocks and applying nitrogen, the subsequent regrowth should look a lot better.

A longer-term solution is when selecting ryegrasses, ensure you select New Zealand-bred ryegrass genetics, since rust tolerance is a key breeding objective for PGG Wrightson Seeds’ plant breeders. New Zealand rust strains can be different to those found overseas, so it is worthwhile choosing grasses that are bred and trialled locally for tolerance to New Zealand rust strains.