Nutritive value and production of yellow bristle grass (Setaria pumila) in Waikato dairy pastures: validating Farmax model assumptions

Yellow bristle grass is a C4 summer-active annual weed prevalent in the upper North Island. Its production and nutritive value was assessed on-farm to validate the assumptions used in a published Farmax model on the impact of yellow bristle grass. The percentage ground cover of yellow bristle grass averaged 9% in a survey of 39 central Waikato dairy pastures assessed each February over 8 years. Yellow bristle grass dry matter content peaked in February. Herbage production of patches of yellow bristle grass was greater than patches of perennial ryegrass when assessed over one yellow bristle grass growing season. Nutritive value was lower for yellow bristle grass than perennial ryegrass over two consecutive growing seasons. Given the prevalence of yellow bristle grass in Waikato dairy pastures, its lower nutritive value and more vigorous growth than perennial ryegrass, it is likely to have a negative impact on the performance of sown pasture species. Findings support the assumptions used in the Farmax model.