Study Points Up Effects of Dietary Protein

1/19/2013 7:00 AM

There have been numerous studies conducted examining dietary effects in dairy cattle diets on ammonia and greenhouse gas emissions from manure.

Penn State’s Alex Hristov and his graduate student Chanhee Lee published their work in the Journal of Dairy Science on the effects of dietary protein concentration on emitting potential of these gases from fresh dairy manure in a controlled environment or following soil application.

With the release of the Air Consent Agreement back in 2005, there has been a lot of focus on air quality.

Ammonia emissions have environmental and human health effects, including eutrophication of surface waters, acidification of ecosystems and fine particulate matter formation in the atmosphere.

The greenhouse gases including methane, nitrous oxide and carbon dioxide have been associated with climate change.

The hypothesis of this Penn State study was that cows fed a high protein diet would emit more ammonia and nitrous oxide, and that greenhouse gas emissions may also be increased due to the availability of nitrogen to fecal and soil microorganisms.

Thirty-six cows were fed two diets that either met the animal’s requirement for protein (16.7 percent on a dry matter basis) or that was slightly protein deficient (14.8 percent).

Fresh fecal and urine samples were collected during the experiment to determine the gas-emitting potential of the manure.

It was no surprise that the average ammonia emitting potential of fresh manure was 89 percent greater for the high-protein diets versus the low protein.

An estimated 39 percent (high protein) and 30 percent (low protein) of the manure nitrogen was lost as ammonia over the 122-hour incubation period.

Decreased manure nitrogen concentration and ammonia losses have been consistently reported as a result of decreased dietary protein in dairy cattle diets. However, the researchers noted that other important factors were not included, such as environmental factors (i.e. temperature), housing type and manure management.

These all greatly affect on-farm emissions. The methane- and carbon dioxide-emitting potential and cumulative emissions were not affected by protein level in this controlled environmental study.

A second part of this experiment was conducted to investigate the effect of dietary protein on the gas-emitting potential of fresh manure following soil application (lysimeters were used).

Composited fecal and urine samples from the two treatments were mixed in a 1.7:1 ratio (feces to urine). The manure mixture was immediately applied at a uniform rate of thickness to soil contained in the lysimeter system.

Emissions of ammonia, carbon dioxide, methane, and nitrous oxide were measured from the manure soil surface at three, eight, 23, 28, 54 and 100 hours after application.

The average ammonia-emitting potential of manure following soil application was about 49 percent greater for the high-protein diet versus the low.

Ammonia gas remained relatively steady during the first 24 hours after the high-protein diet manure application and decreased thereafter.

The emitting potential for the low-protein manure sharply decreased during the first 24 hours of manure application.

The gas-emitting potential difference between the two manures remained large at 50 hours. This can be explained by the proportionally greater concentration of urinary urea nitrogen in the manure produced from the high-protein diet versus the low-protein.

Methane and carbon dioxide emitting potential from the manure-amended soil tended to increase on the manure produced from the low-protein diets. This most likely stemmed from the greater addition of fecal matter with the low-protein diet manure compared with the high-protein.

In the short term experiment, nitrous oxide was not affected by dietary protein level.

The authors emphasized that this experiment was conducted in a controlled environment and other effects related to storage and the environment were not accounted for.

However, it does point out that management strategies have to be evaluated on the whole farm system and that certain best management practices to control one gas may end up increasing a different gas at another point in the farm operation.

Virginia “Ginny” Ishler is a nutrient management specialist and manager of Penn State University’s dairy complex.

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