Nitrate leaching and residual effect in dairy crop rotations with grass–clover leys as influenced by sward age, grazing, cutting and fertilizer regimes

Abstract

Intensive dairy farming, with grass-arable crop rotations is challenged by low N use efficiency that may have adverse environmental consequences. We investigated nitrate leaching and N fertility effects of grass–clover leys for five years in two organic crop rotations with different grassland proportions (33 and 67%) and five grassland managements in terms of cutting, grazing, fertilization and combinations thereof. In grass–clover, the combination of fertilization and grazing caused excessive leaching (average 60kg Nha−1) but leaving out either fertilization or full-time grazing substantially reduced leaching losses to on average 23kg Nha−1. There was no linear relationship between sward age and nitrate leaching. The annual N surplus of the grasslands was only weakly related to nitrate leaching (R2=0.05, P<0.01). The residual effect of grassland cultivation was highest following grazing and fertilization and lowest without fertilization or cutting-only management and was not influenced by grassland age. The arable part of the mixed crop rotation was nitrate leaky where crop coverage in autumn was insufficient. Nitrate leaching following the crops may roughly be divided into four groups: (1) low leaching (<10kg Nha−1) consisting of the barley wholecrop undersown with Italian ryegrass, (2) medium–low leaching (10–20kg Nha−1) consisting of barley with grass–clover undersown and the 1-yr-old grasslands, (3) medium leaching (25–50kg Nha−1) consisting of barley–pea and 2-4-yr-old grasslands with the 2-yr-old leaching the most (36–46kg Nha−1), and (4) high leaching (>50kg Nha−1) with lupin and maize, where especially maize was consistently high in all five years (average 81kg Nha−1). Great care should be taken during all phases of the dairy crop rotation where grasslands cause considerable build-up of fertility. With due care and the best management practice, nitrate leaching losses may be reduced to low levels.