A mathematical definition of MEY is illustrated for grain

yield of irrigated corn in figure 1 where the production input is

nitrogen (N). Maximum yield and MEY for a cost/price ratio of 0.58

are shown. Nitrogen rate and MEY for a cost/price ratio of 0.58

are 150 lb/A and 203 bu/A, respectively. The MEY for a cost/price

ratio of 1.0 is 120 bu/A and nitrogen rate is 45 lb/A. To

determine MEY for any cost/price ratio, construct a horizontal line

from the cost/price axis to the curve representing the cost/price

function, then construct a vertical line from the C/P curve to the

yield curve, finally construct a horizontal line from the yield

curve to the yield axis, the resulting value is the MEY. Nitrogen

rate is equal to the distance between the yield axis and the

vertical line connecting the two response curves.

 Some input costs remain fixed regardless of yield goal. These

costs include equipment, land, pesticide, etc. Variable costs such

as nitrogen, phosphorus, potassium, seed, and irrigation change

with yield goal. Nitrogen is usually the factor that limits yield

of corn most. Therefore, when N rate is reduced other inputs can

be reduced in proportion to the N reduction. Consequently, all

variable input costs can be expressed in terms of cost per pound of

N.

 Magnitude of profit margin, which may be positive or negative,

depends on the difference between total input costs and total

output returns. Remember, when profit is negative low or high

input agriculture is not sustainable. The effect of increasing N

cost on profit from irrigated corn at $2.00/bu is shown in table 1.

As expected, the cost/price ratio increased as the cost of N