It could possibly depend on the quantity eaten. Koromiko leaf tips were famously used by NZ troops in the war to help stop diarhhoea, But since it is a common shrub in regenerating shrublands frequented by cattle in places like the East Cape I suspect that it is not particularly toxic to cattle. Like any stock health issue - consult a vet if there are symptoms of ill health. Let us know if koromiko is shown to be toxic in any way and we can update information on the website
Mike