The environmental impact of dairy continues to be scrutinized, and the front door and back door of the cow are connected. The rations you formulate have a tremendous impact on the amount of manure your cows produce and your dairy’s environmental impact.

“We need to start taking into account the environmental costs of what we’re feeding and the cost to meet environmental requirements,” says Virginia Ishler, nutrient-management specialist at PennStateUniversity.

According to work done at OhioStateUniversity, it is possible through dietary strategies to manipulate rations to reduce manure production by 15 percent, without negatively affecting milk production.

Using a cow that produces 75 pounds of milk and 150 pounds of manure (liquids and solids) per day as the baseline, this reduction equates to 22.5 pounds less manure per cow per day and about 8,200 pounds per year per cow. A significant reduction in manure volume is seen when you multiply this figure across an entire herd. For an example, a 100-cow operation would see a reduction of 821,250 pounds or about 411 tons of manure. 

Evaluate these seven areas when formulating rations to ensure lower manure production:

1. Corn silage vs. alfalfa

The ratio of corn silage to alfalfa is the dietary factor that has the greatest effect on manure output.

As the percentage of corn silage in the ration increases, urine output decreases substantially. This results in a significant decrease in the volume of manure.

“Our work shows that a 10-percent increase in corn silage results in a decrease of about 4 pounds of manure per cow per day,” says Bill Weiss, dairy nutritionist at OhioStateUniversity.

Cows fed 100 percent of the forage as alfalfa produce twice the amount of urine per day as cows fed 100 percent corn silage diets. The difference, Weiss believes, is due to the potassium concentration between the two forages. “Corn silage almost always has a lower concentration of potassium than alfalfa, so as corn silage increases and alfalfa decreases, the amount of potassium in the diet decreases.”

Changing the ratio of corn silage to alfalfa has no effect on milk production if the ration is balanced accordingly.

2. Crude protein

Crude protein levels in the ration affect the volume of manure produced.

For every 1-percent decrease in crude protein, there is a 2-pound decrease in manure production per cow per day.

“Reducing crude protein by 1 percent also represents about 25 tons of urea equivalents per year for a dairy milking 1,000 cows,” says Alejandro Castillo, farm adviser with the University of California Extension Service.

Diets should contain adequate, but not excessive, levels of protein, says Ishler. “Feeding 17.5 percent crude protein in a diet has been a safety net. It has been cheaper and less complicated than formulating for metabolizable protein; however, it’s no longer just about the cost of production anymore. We now have to think about environmental costs,” Ishler says.

“If you formulate for metabolizable protein, you can successfully feed low-protein diets of 15 to 16 percent and still maintain 100 pounds of milk,” she adds.

3. Fiber

As the concentration of neutral detergent fiber (NDF) increases, manure output usually increases.

“Because NDF concentration is negatively correlated with the concentration of starch in diets, feeding higher-starch diets will reduce manure output,” Weiss says. “And, because changes in starch and NDF concentrations are usually confounded, we cannot determine whether the effect is caused by fiber or by starch.”

A 1-percent increase in NDF concentration will increase manure output by 0.5 to 1 pound per day. Conversely, a 1-percent increase in starch decreases manure output by a similar amount.

“The overall effect of varying NDF concentrations on manure production is usually less than 10 pounds per cow per day,” Weiss says.

4. Dry matter intake and digestibility

Dry matter intake (DMI) and digestibility are directly tied to manure output.

“Our research shows that increasing DMI from 35 to 40 pounds per day increases manure output by 2.7 pounds per pound of dry matter. But when we increased DMI from 55 to 60 pounds per day, we saw an average increase of 3.5 pounds per day of manure per pound of increased dry matter,” says Weiss.

As intake increases, digestive efficiency tends to decrease because feed passes through the cow’s system at a faster pace. “A small decrease in digestibility has a major impact on manure production,” notes Weiss. A ration change from 85 percent to 80 percent digestibility results in 33 percent more solids in the manure.

You would expect slightly lower digestibility at higher intakes, resulting in more manure per pound of intake. However, intake and milk production are correlated. On average, high-producing cows eat more than low-producing cows. “You should not restrict intake so that cows produce less manure, as it will also likely reduce milk production,” says Jud Heinrichs, professor of dairy science at Penn State University.

Instead, focus on the digestibility of the diet. “Feeding highly digestible diets results in high milk production at reasonable intakes with reasonable rates of manure production,” says Ishler.

To evaluate diet digestibility, look at feed efficiency — pounds of fat-corrected milk per pound of dry matter. For most situations, herd average feed efficiency should be around 1.5 to 1.6. “Improved feed efficiency also means more money and less manure,” notes Castillo.

5. Evaluate byproducts

“What goes in must come out,” says Ishler. “By-products may be cheaper to feed, but may not be as digestible. The environmental costs may outweigh the savings on feed.”

For example, the digestible NDF for cotton hulls is 17 percent versus beet pulp at 39 percent and soy hulls at 54 percent. “An ingredient that is highly digestible will be used by the animal for milk production. Ingredients that are indigestible are just going to go through the animal. It is how you balance these ingredients to maintain a cost-effective ration, a healthy diet for the cows and minimize both volume of manure and nutrients excreted,” she adds.

6. Look at mineral load

Look at the mineral load in the ration. Keep the level of these ingredients as close to the cows’ requirements as possible to cut back on manure production. For example, over-feeding sodium and potassium increases water intake, which raises manure output.

You don’t want to be deficient. But over-feeding minerals only changes manure, not the amount of milk produced.

7. Forage quality

Very good high-quality and highly digestible forages can make a big difference on the amount of manure produced.

Increasing forage quality will almost always reduce the amount of manure produced per pound of milk produced. “The higher-quality forage is digested and those nutrients are converted into milk rather than manure,” says Weiss.

“We would expect manure output to increase by about 10 to 12 pounds per day when cows consume lower-quality, less-digestible forage,” he adds.

Finally, evaluate how feed is handled on farm, monitoring dry matter on all ingredients. Make sure dry-matter percentages are adjusted routinely, scales are working properly and each cow receives the prescribed ration.