Impact of yellow bristle grass (Setaria pumila) on Waikato dairy pasture performance: validating Farmax model assumptions

Yellow bristle grass is a C4 summer-active annual weed prevalent in the upper North Island. 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 clumps of yellow bristle grass was greater than clumps perennial ryegrass when assessed over one 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 a previously published Farmax model estimating the impact of yellow bristle grass on farm profit.