Fertilizer Facts: September 1995, Number 7

Grass Response to N Fertilizer

Jeff Jacobsen, Scott Lorbeer, Harold Houlton and Gregg Carlson
Dept. of Land Resources & Environ. Sci. and Northern Ag Res. Center
Montana State University

Grass yield and quality response to nitrogen (N) fertilizers is dependent on plant available nutrients, water, climate and other agronomic factors. Fertilizer application can be a key management tool, if soil test levels are less than optimum, economics are favorable, and the environment will not be adversely impacted.

Two dryland grass experiments were conducted at the Northern Agricultural Research Center (48^o 30' N; 109^o 47' 30" W) at Havre, MT, to evaluate grass specie response to a single N application. Both used ten common grass species (basin wildrye (BWI), beardless wheatgrass (BWH), crested wheatgrass (CWH), green needlegrass (GNE), "Greenar" and "Oahe" intermediate wheatgrasses (GIW & OIW), pubescent wheatgrass (PWH), Russian wildrye (RWI), thickspike wheatgrass (TWH and western wheatgrass (WWH)) that were seeded in the early spring and fertilized in the fall. Nitrogen fertilizer (34-0-0) was applied in Trial 1 at 0, 50 and 100 lbs N/a and in Trial 2 at 0, 100, and 200 lbs N/a. Dry matter production was measured after the first year of production and annually for four years.

Cumulative Production

In both trials, N fertilization increased forage production (Table 1). In Trial 1, averaging all species, cumulative yield over the 4 years increased from 5384 lbs/a for the check to 6580 lbs/a with 50 lbs N/a, and to 6867 lbs/a with 100 lbs N/a. If only the four greatest producing grasses are considered, cumulative yield increased from 6216 lbs/a for the check to 8148 lbs/a with 50 lbs N/a, and to 8427 lbs/a with 100 lbs N/a.

Table 1. Cumulative 4-year yields for two trials.

In Trial 2, averaging all species, cumulative yield increased from 3800 lbs/a for the check to 6924 lbs/a with 100 lbs N/a, and to 8389 lbs/a with 200 lbs N/a. Again, looking only at the four greatest producers, cumulative yield increased from 4567 lbs/a with the check to 8606 lbs/a with 100 lbs N/a, and to 10411 lbs/a with 200 lbs N/a.

A single N fertilizer application continued to show a response in some cases four years after application. Apparently, if growing conditions are poor, residual N remains in the soil/plant system, increasing production in subsequent, more favorable years. The N response was greater in Trial 2 due to the more favorable timing of precipitation (Table 2).

Table 2. Comparison of annual yields in two trials at two rates.

N Response by Species

The individual species responded differently to N applications. Crested wheatgrass, both intermediate wheatgrasses and pubescent wheatgrass produced large yields and consistently responded the most to N fertilization.

Fertilizer Use Efficiency

A fertilizer use efficiency (FUE) value was calculated as pounds of grass greater than the check per pound of applied N. In Trial 1, grasses treated with 50 lbs N/a were more efficient than 100 lbs N/a (23.9 and 14.8 lbs grass/lbs N, respectively), and in Trial 2, 100 lbs N/a produced more efficient response than 200 lbs N/a (31.2 and 22.9 lbs grass/lbs N, respectively).

Economic Implications

A simple cost-benefit analysis was made using prevailing values ($210 per ton N, $40 per ton hay). The fertilizer cost (without application cost) is $11.41, $22.82 and $45.64 per acre for 50, 100, and 200 lbs/a applications, respectively. In order to break even, a cumulative production increase of 570, 1140 and 2280 lbs/a is needed to offset just the fertilizer cost of 50, 100, and 200 lbs N/a, respectively. More simply, an 11.4 lb forage production increase is needed for every lb N applied to pay for the applied N.

In Trial 1, 50 lbs N/a produced this minimal level in 8 of 10 species with pubescent wheatgrass returning the most hay per fertilizer dollar, and thickspike wheatgrass the least. With 100 lbs N/a, 7 of 10 species produced more than the economic break even point with "Oahe" intermediate wheatgrass the most economical, and western wheatgrass the least. In Trial 2, all 10 species at both fertilization levels produced economic responses, with crested wheatgrass producing the most hay per fertilizer dollar and basin wildrye the least at both N levels. Clearly, to produce enough hay to justify the cost of fertilizer, the species composition, fertilizer rate, market factors and application costs needs to be carefully considered.

Fertilizer Facts

• Grasses respond to fertilizer N applications with the magnitude of response species dependent
• Crested wheatgrass, "Greenar" and "Oahe" intermediate wheatgrasses and pubescent wheatgrass were consistently the highest producers
• These four grasses are most likely to give increased hay value for every invested fertilizer dollar
• During dry seasons, N can remain in the soil or plant system for use in later, more favorable seasons
• N fertilizer can remain in soil and boost production for several years, and therefore fertilizer may not need to be applied each year