Grazing intensity and nitrogen fertilization effects on biomass and morphology of black oat roots in an integrated crop–livestock system

Grazing stimulates root and forage mass of perennial pastures, but few studies have addressed the effects of grazing on annual pastures which are often grown in short periods within integrated crop–livestock systems (ICLS). In these pastures, N fertilization is a common practice to increase forage mass, but its effects on root morphology are unclear. In a field experiment, we evaluated how grazing intensities (ungrazed, moderate, and heavy grazing) and N fertilization rates (0, 75, and 150 kg N ha⁻¹) affected morphological attributes of oat (Avena strigosa Schreb.) roots and forage accumulation in ICLS in Brazilian subtropics. Forage mass was evaluated in each grazing cycle and roots were sampled to a 30-cm depth at the end of the grazing season in 2 years. Heavy and moderate grazing reduced (p < 0.05) forage accumulation by 15%–37% compared to ungrazed (4867 kg DM ha⁻¹) in the first year, characterized by extended periods of water deficit, but in the second year forage accumulation from grazed and ungrazed was similar (p > 0.05), averaging 4273 kg ha⁻¹. N fertilization increased forage accumulation in both the years, without differences between 75 and 150 kg N ha⁻¹ in grazed treatments. Grazing and N rates mainly reduced root diameter and increased root N concentration. Increasing N fertilization rates increased root length but it was more expressive in the first year. Nitrogen fertilization with 75 or 150 kg N ha⁻¹ on moderately grazed oat is an alternative to increase forage accumulation as it stimulates fine root development and increase N uptake, mainly when adverse climatic conditions can affect pasture growth.