Addressing nutrient co-limitations improves productivity in tropical grazing systems: Short-term field evidence

Context Fertilization of tropical pastures in Latin America is limited and predominantly based on urea as the sole nitrogen (N) source. This contrasts with the widespread multi-nutrient limitations of highly weathered soils, which may restrict the effectiveness of N-only fertilization. However, field-scale evaluations of alternative fertilization strategies under grazing conditions remain limited. Methods We evaluated fertilization strategies combining additional nutrients and seasonally adapted N sources (hereafter referred to as Opt treatments) for their effects on forage production and cattle productivity under grazing conditions, compared with conventional urea fertilization in Urochloa humidicola pastures. A replicated field experiment was conducted in the Eastern Plains of Colombia, comparing two doses of the Opt fertilization strategy (Opt-100 and Opt-75) with equivalent urea-based treatments and an unfertilized control. A modified treatment excluding potassium (K) and partially reducing phosphorus (P) inputs (Opt-Nutrients) was also included. In addition, a complementary miniplot experiment was conducted to assess short-term nitrous oxide (N₂O) emissions. Results The Opt-100 treatment achieved a daily cattle weight gain of 4.81 kg live weight ha⁻¹ day⁻¹, nearly doubling the control and exceeding the Urea-100 treatment by 40%. Because stocking rates were adjusted according to the expected productivity level of each treatment, these differences reflect the performance of integrated fertilization-management strategies under grazing conditions rather than the isolated effect of fertilization alone. Daily weight gain reached 1.19 kg animal⁻¹ day⁻¹ in Opt-100 compared with 0.88 kg animal⁻¹ day⁻¹ in the control, and animals reached the target weight one rotation earlier (196 vs 224 days). This response is consistent with alleviation of nutrient co-limitations, as Opt-Nutrients and Urea-100 showed similar gains, indicating that nutrient balance rather than N source drove the response. Increasing fertilization rates resulted in greater productivity gains in the Opt than in the urea-based treatments. Forage biomass accumulation was also higher under Opt fertilization compared with the unfertilized control, although differences were partly moderated by higher stocking rates. Opt-100 and Opt-Nutrients treatments did not consistently increase short-term N₂O emissions relative to the control under the experimental conditions. Conclusions and implications These results provide field-based evidence that addressing nutrient co-limitations, rather than increasing N inputs alone, is critical to improving the productivity of tropical grazing systems. Balanced fertilization, combined with appropriate stocking-rate management, represents a practical strategy for pasture intensification under grazing conditions, with potential environmental benefits.