Review: A field trial evaluated the effect of differential irrigation after application of 40 kg of urea/ha. The additional 30 mm irrigation treatment increased grass height by 73%, dry matter production by 41%, and generated an additional US$75/ha in 30 days. The extra irrigation counteracted the need for more nitrogen, improving use efficiency and grass growth.
Timely differential irrigation over nitrogen fertilizer application can improve nitrogen use efficiency (NUE) and increase grass growth. An experiment was carried out in a grazing field to evaluate the effect of applying 20 mm of additional irrigation after the application of 40 kg of urea per hectare.
Treatments
Two irrigation treatments were tested in 45 plots of 1 hectare each: Control treatment: Application of 40 kg of sulfur urea per hectare with 10 mm of post-application irrigation, and water balance irrigation is maintained.
Proposed treatment: Application of 40 kg of sulfur urea per hectare with a total irrigation of 30 mm, 10 mm before application and 20 mm after application, making three steps of 10 mm each. The 30 mm applied regardless of the water balance.
Measured variables
The following variables were measured 30 days after treatment:
•Vegetation heigh
•Botanical composition: proportion of grasses and other species (mainly legumes and chicory)
•Dry matter
Results
The results show significant differences between treatments for all variables. The proposed treatment produced a higher vegetation height, a higher percentage of grasses and a 41% increase in dry matter production compared to the control treatment.
Height (cm), % Grasses, % Others, % Dry matter, Dry matter (kg/ha)
Control 23,3; 60; 40; 27,5; 2.680
Proposed 40,3; 85; 15; 30,1; 3.791
Height was 40.3 cm versus 23.3 cm for the control plot, and the percentage of grasses was 85% versus 60% for the reference plot. It is likely that the increase in the growth of grasses is due to their greater capacity to take advantage of water and nutrients (especially applied nitrogen). The dry matter yield was 1,111 kg/ha higher in the proposed treatment. It is estimated that this could give an extra production of 46.3 kg of meat per hectare, which generates an economic benefit of US$75 per hectare, considering the value of the kg produced similar to the system in which the trial is located.
In conclusion, a greater response was observed in grass growth with the differential irrigation treatment. This could be due to a greater efficiency in the use of applied nitrogen. For future experiments, it would be interesting to include treatments with higher nitrogen doses (e.g. 80 kg/ha) to evaluate whether the benefits of differential irrigation are maintained when higher doses are applied. Differential irrigation in timing of urea application could offset the need to apply more nitrogen, which would have a large impact on pasture irrigation management.