Many factors impact land requirements including:
1) Dairy feeding program: Feeding excess protein or P increases N and P excretion.
2) Animal performance: Higher-producing cows excrete more manure; 90 lb milk/day was assumed in the example below.
3) Crop yields: A 24-ton/acre and 6-ton/acre yield for corn silage and alfalfa was assumed in the example below.
4) Use of manure on legume crops: Lack of economic return from manure N and possible damage to alfalfa may discourage use of manure on alfalfa by some dairies.
An additional factor is whether a nutrient plan is based on nitrogen (N) or phosphorus (P). For a crop rotation that is predominantly corn silage and alfalfa hay, the approximate land requirement per lactating cow is shown below for a manure system that conserves N and for three distinct dairy rations:
Manure Applied to Corn Only: N / P-based rates
Current Recommendations (18.5% CP & 0.33 %P): 3.1 / 3.1 acres per cow.
Ration with 30% DGS (20.4% CP & 0.45% P): 3.6 / 3.8 acres per cow.
Ration from 10 years ago (18.5% CP & 0.5% P): 3.1 / 4.1 acres per cow.
Manure Applied to Corn and Alfalfa: N / P-based rates
Current Recommendations (18.5% CP & 0.33 %P): 1.7 / 1.7 acres per cow.
Ration with 30% DGS (20.4% CP & 0.45% P): 1.9 / 2.0 acres per cow.
Ration from 10 years ago (18.5% CP & 0.5% P): 1.7 / 2.2 acres per cow.
30% DGS: 30% inclusion of distillers grains with solubles on a dry matter basis.
Several observations result from this information. First, a traditional rule of thumb of 1 acre per cow is possibly too simplistic for modern dairy cattle. Second, as the concentration of P in the dairy ration has decreased, N often becomes the limiting nutrient for manure application, and so an N and P-based application rate is often similar (this is true for nitrogen-conserving system only and assumes that manure application never exceeds N requirement). Third, use of DGS in the diet increases both N and P excretion and the resulting land required for managing manure.
Tools and fact sheets to assist dairy nutrient planning can be found at eXtension LPE Feed Management. To determine land requirements for your own farm, you may want to enter your own farm-specific information into a Nutrient Inventory spreadsheet.
Author: Rick Koelsch, University of Nebraska