Adapted by Deanne Meyer, Livestock Waste Management Specialist
Water, water everywhere! That was the winter of 2017. Water’s been flowing in irrigation ditches and canals since early winter. It’s been so long since surface water has been available, it’s good to review nutrient management strategies. Many dairy operators will use this abundance of surface water to dilute ponds during the irrigation season. It’s important to manage this magnificent opportunity and potentially sample your liquid manure more frequently.
Here’s why you may want to sample your liquid manure more frequently than once a quarter if you’re using surface water to dilute your pond. Liquid manure nutrient concentration is a function of the amount of manure that is collected and the amount of water than ends up in the liquid system. The liquid manure nutrient content decreases when the same amount of manure is collected and more water enters the lagoon. Use of feedline or holding area soakers, or addition of irrigation water to lagoons usually results in a decrease in liquid manure nutrient content. If you overestimate liquid manure nutrient content, then you end up providing fewer nutrients for your growing crop.
Let’s look at an example. You’re planning to apply 3 inches of liquid manure per acre (nitrogen concentration is 450 mg/l) over the course of summer. This should apply about 306 lbs of nitrogen (N) per acre to your corn crop. [Note—with expected yields of 225 lbs of N per acre, the 306 lbs of N applied per acre is roughly at the targeted 1.4 N applied to N removed ratio]. That’s a reasonable target. If the first inch of liquid manure applied is near the date of sampling, you have confidence that you applied about 100 lbs of N per acre. After adding surface water to your lagoon, the concentration of N will drop. If it drops to 250 mg/l before the remainder of the applications, the other 2 inches of liquid manure will contain less N. Two inches of water applied per acre at 250 mg/l of N provided 113 lbs of N per acre instead of your anticipated 204 lbs of N per acre. This difference ends up shorting your crop of N for growth. Your target value of 306 lbs of N applied per acre is missed when only 213 pounds of total N are applied per acre (the 100 lbs of N in the first inch of liquid manure plus the 113 lbs of N in the remaining two inches of liquid manure applied per acre). Since your expected yields were in the 225 lbs of N per acre range, the diluted manure application results in under application of N for your crop.
| Importance of having representative samples of liquid manure to use in determining nutrient applications. Pounds of nitrogen (N) in liquid manure when the pond is diluted between first application and remaining applications of liquid manure. Estimated total N to apply is near 315 lbs of N/acre. |
| What I thought I had | What I ended up with | |
| First inch of liquid manure applied | 102 lbs of N applied/acre | 102 lbs of N applied/acre |
| Next two inches of liquid manure applied | 204 lbs of N applied/acre | 113 lbs of N applied/acre |
| Total pounds of Nitrogen applied per acre | 306 lbs of N applied/acre | 215 lbs of N applied/acre |
For a mere cost of another sample of liquid manure, you would be more informed about the nutrient content of your liquid manure and be able to more closely apply N to meet crop needs.
Link to PDF Version: Managing Lagoons When There's Water