Does cattle grazing of dual-purpose wheat accelerate the rate of stubble decomposition and nutrients released?

Decomposition and nutrient release of winter annual forages in integrated crop-livestock systems could be affected by the resultant alterations in structure and quality of residues caused by grazing, but little information is available to test this hypothesis. Information on residue dynamics is needed to synchronize nutrient release from cover-crop litter to cash-crop nutrient demand and to overcome potential deficiencies with appropriate fertilization strategies. We evaluated the influence of different grazing period lengths (0, 21, 42, 63, 85, and 105 days) of dual-purpose wheat (Triticum aestivum L.) pasture on the release rate of N, P, K, Ca, and Mg from decomposing wheat straw in a field experiment at the Agronomic Institute of Paraná, Brazil. Wheat litter decomposition and nutrient release were assessed with litter bags placed at the soil surface in a completely randomized design with seven collection periods (0, 15, 35, 63, 98, 139, and 196 days) during the subsequent growth of soybean [Glycine max (L.) Merr.]. Wheat residue decomposition and nutrient release followed an exponential function with time. Enhanced decomposition of wheat residue with previous grazing did occur, but the influence was relatively small (range of 6–11% greater release, except for 4% inhibition of K). In integrated crop-livestock systems, the effect of residual forage mass at the cessation of the grazing season (structure) would have much greater impact on nutrient cycling than composition of plant litter (quality). Nutrient cycling via animal excrement was not investigated, but was considered another significant mechanism for enhanced nutrient cycling with integrated crop-livestock systems. Grazing of wheat tended to have negative consequences on soybean grain yield, declining 9 kg ha−1 for every day of grazing (i.e. 19% lower yield at 105 days of grazing compared with no grazing). Minor changes in nutrient release dynamics can be expected with grazing of winter cover crops, but these changes are important for managing nutrients in integrated crop-livestock systems.