Utilizacion de urea liquida (ULB-35) en produccion de forrajes en Puerto Rico

Four experiments were conducted on commercial farms, to quantify forage yields and nutritional contents of pastures used for hay with the application of liquid urea (ULB-35RTM). Liquid material was applied through the center pivot irrigation system (Lajas-1), travelling sprinklers (Sabana Grande and Lajas-2) or by aerial fertilization equipment (Arecibo). In Lajas-1, the total dry matter (DM) yield (6 months) was highest for 15-5-10 (N-P 2O5-K2O) complete fertilizer (168 kg N/ha), followed by the middle (74.7 kg N/ha), high (112 kg N/ha) and low (37.3 kg N/ha) levels of ULB-35RTM, with 10,429, 7,158, 5,776 and 4,534 kg MS/ha, respectively. In Sabana Grande, DM forage yield differences at 6 months were not found between ULB-35RTM (9,743 kg DM/ha) and 15-5-10 complete fertilizer (9,078 kg DM/ha) when both were applied at 112 and 84 kg N/ha, respectively, but both treatments were higher than ammonium sulfate (6,666 kg DM/ha) applied at 84 kg N/ha. In Arecibo, differences were not found for forage yields, protein and nitrogen-use efficiency, after applying 56 kg N/ha/application as ammonium sulfate and three concentrations of ULB-35 RTM (7.5, 3.0 and 1.5% N). However, ULB-35RTM was 50% more economical than ammonium sulfate. In Lajas-2, differences were not found for forage yield and protein, between treatments ULB-35RTM (112 kg N/ha/application), ammonium sulfate and granular urea (when both were applied to 56 and 112 kg N/ha/application). However, nitrogen-use efficiency was 90% higher for low levels (56 kg N/ha), than with high levels (112 kg N/ha). A quantitative relationship between chlorophyll meter SPAD-502RTM and N concentration in forage tissue was developed.To complement the agronomic studies, four experimental NH3-N volatilization assays were performed (in laboratory and field conditions). The rate of volatilization increased with the soil pH (5.43 to 6.45), decreased with time (80.8% of volatilization occurred during the first seven days), and was higher for granular urea, in relation to other nitrogen sources (ULB-35RTM and ammonium sulfate). Ammonium volatilization with ULB-35RTM was similar to ammonium sulfate and, in most cases, these did not differ from the control treatment (without N). Volatilization was 9.0% of the 75 kg N/ha applied. The use of the urease inhibitor Agrotain RTM reduces NH3-N volatilization from ULB-35RTM by 54.0%.