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Nitrogen (N) plays a key role in pasture productivity and is usually the most limiting nutrient in grasses. Adding commercial N fertilizer is one approach to boost productivity, but there are several formulations. Which one provides the best yield and economic returns on irrigated pasture?

In 2019, we conducted an experiment in Chesterfield, Idaho on irrigated meadow brome (Bromus biebersteinii) pasture. In this trial, we tested four different N formulations at varied rates and compared these to no N addition. The following N fertilizers were applied: ammonium nitrate, urea, SuperU, and Environmentally Safe Nitrogen (ESN). Rates were 0, 60, and 120 units of N per acre, applied on May 9th. Within three days of application, the site received an estimated ½ inch of rain. Throughout the summer, the site was irrigated, and the treatments were harvested and weighed, with the last cutting occurring September 27th.

Results indicated that all N sources produced similar yields, except for the “ESN 60” treatment, which produced significantly less. The “zero” treatment (no N addition) produced 850 pounds of dry matter per acre. When 60 units of N were applied (regardless of source, besides ESN 60), yield jumped to roughly 2100 pounds of dry matter per acre. When N was increased to 120 units per acre, there was another small increase, regardless of N source. This incremental increase was only equivalent to about 100 – 300 pounds of dry matter per acre.

While the source of N had little effect on yield, the cost of the various fertilizers had a significant effect on economic return. Urea 60 had the best economic return at about 42 pounds dry matter per dollar spent on N. Ammonium Nitrate and SuperU at 60 units were close behind at 34 and 32 pounds dry matter respectively, per dollar spent on N.

From this experiment it can be inferred that urea is a great choice for irrigated pastures if irrigation is applied or precipitation is received soon after fertilization. Unfortunately, urea is prone to volatilization and denitrification losses if is not incorporated into the soil. A product like SuperU might be a good alternative if you’re not able to irrigate or time application prior to a precipitation event. SuperU is a form of urea fertilizer that has stabilizers added to protect it from losses.

This study answers a small part of the quest to produce quality, efficient pasture. If you would like more in-depth information regarding this study, see: Progressive Forage Magazine, April 1, 2021.

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