Wheat - black-grass competition is a non-zero-sum game influenced by root growth

Interactions with neighbouring plants have profound effects on plant growth, especially in crop-weed interactions, which can cause major crop-losses in agricultural systems. Black-grass (Alopecurus myosuroides) is the most problematic weed for UK agriculture, causing dramatic yield losses in winter wheat, but the basis of its competitive advantage is not clear. We aimed to fundamentally reappraise the nature of wheat - black-grass competition. Here, we show that black-grass is slow to establish, and requires long periods to demonstrate any competitive advantage over wheat. We show that black-grass grows significantly faster under winter conditions than wheat, and has much more vigorous root growth, collectively suggesting that prolonged root growth over winter is key to black-grasss competitive advantage over wheat. We tested the competitiveness of a range of wheat and barley germplasm against black-grass in three distinct environmental set-ups, and show that competitiveness under controlled conditions reflects competitiveness under field conditions. In controlled conditions, we identified significant variation in crop varietal competitiveness against black-grass, in both ability to suppress black-grass growth, and to continue to grow in the presence of black-grass (tolerance). We further identified that this competition is not a zero-sum game, with loss in crop biomass not necessarily equal to gain in black-grass biomass. Finally, we found that the black-grass suppression, but not tolerance, is correlated with crop root growth, supporting our root growth hypothesis for black-grass competition. Overall, our results suggest that breeding for increased root growth and/or winter growth rate can create more competitive wheat cultivars for grassweed suppression.