There was a time when a dairy farmer’s biggest concern about manure was whether the gutter cleaner was functional, if the tractor might break down or if there was a spare hand to send to the field with a spreader full of manure. The chemistry of that manure was probably not at the top of the farmer’s mind.
Although farmers have been using manure to their advantage for centuries, increased knowledge about the impact of manure on the environment has led to science-based research and legislation that dictates how manure must be handled.
“The feed management conservation practice was originally developed 10 years ago to address water quality issue to reduce nutrients going from animal manure into the waters of the United States,” said NRCS senior economist Dr. Glenn Carpenter. Carpenter says when the feed management standard was updated three years ago, new feed management allowances addressed air quality issues and pathogen management.
USDA-ARS soil scientist and agroecologist Dr. Mark Powell explains that two legislative initiatives — The Clean Water Act and the Clean Air Act — are the reason behind feed management and how management affects the environment.
Powell says while some nutrition trials examine dietary impacts on milk production and reproduction, his research focuses on how the dairy diet influences the chemistry of manure and how that chemistry impacts the environment. Powell conducted research on confinement farms, but says the results apply to pasture-based dairy farms. From the late 1990s and through 2004, research focused on manipulating dietary phosphorus to influence the phosphorus chemistry of manure and how the cow’s diet can impact phosphorus runoff following manure application.
“In 2004, we started to get more into air emission issues, including ammonia loss and carbon dioxide, methane and nitrous oxide emissions,” said Powell. “We’ve been looking at how dietary crude protein, forage types and the how the addition of tannin and other dietary components impact the chemistry of manure.”
Powell says the typical dairy diet contains about 50 percent forage, 20 to 25 percent each of protein supplements and grain, and a small amount of minerals and vitamins. “Initial phosphorus research was done on the mineral phosphorus being applied to the ration,” said Powell, “mostly under the recommendation of feed consultants and veterinarians. That mineral phosphorus has an impact on the amount of phosphorus that comes out in the feces.” Powell pointed out that most phosphorus is excreted through manure, with very little in urine.
Major dietary nitrogen sources are from forage and protein supplements, which both impact nitrogen levels in feces and urine. “The type and amount of forage and protein supplement impact the relative amount of nitrogen that is excreted in the feces as microbial (or bacterial) nitrogen,” said Powell, “which is nitrogen that comes from the cow versus the amount of nitrogen that is associated with undigested fiber nitrogen that comes out in feces.” Powell says the fibers visible in manure have passed through the gut undigested, and have a distinctly different function in the soil system compared to the nitrogen in the feces that comes from microbes and bacteria.
Urea nitrogen, or nitrogen in the urine, is the main source of ammonia and nitrous oxide emissions. The amount of forage fed, the type of forage and the type of protein supplement can impact nitrogen emissions and nitrogen cycling in the soil.
The major impact of carbon is influenced by the amount of grain and type and amount of forage, and affects structural and non-structural carbohydrates in feces.
The form and amount of dietary crude protein (CP) fed to dairy cows affects the nitrogen in feces, nitrogen in urine, and ammonia and nitrous oxide emissions. Powell noted some studies examined fecal nitrogen mineralization in soil and plant uptake. “Twenty to 35 percent of feed nitrogen ends up in milk,” said Powell. “The rest of nitrogen goes to manure. Everything relates back to the type and amount of protein.”
As more nitrogen is fed, the response of the cow to nitrogen levels off; which is similar to what occurs in soil over time. “Like any biological response, the first couple increments of feed nitrogen are good,” said Powell. “The animals respond to it, but that levels off quickly and feed nitrogen use efficiency declines and more nitrogen comes out in manure.”
Powell says management impacts the amount of nitrogen that goes into milk and the amount that goes into manure. “If farmers feed balanced rations, much more feed nitrogen goes into milk,” he said. “Feeding a TMR results in even more feed nitrogen in milk.” Most excess nitrogen is excreted in urine, with a small amount excreted in feces.
With fresh stored manure, there is more than a two-fold loss difference in ammonia emissions if the manure that is being spread comes from cows fed a high crude protein diet versus cows fed a crude protein adequate diet. Powell says ammonia emissions can be reduced by as much as 50 percent by feeding a balanced diet with the correct levels of crude protein.
“Manure from cows fed a high-protein diet has more plant-available nitrogen than manure from cows fed a low crude protein diet,” said Powell. “We’re getting more ammonia emissions from the high crude protein diet, but when ammonia emissions are finished, what’s left in the soil is more plant-available nitrogen. By reducing dietary crude protein, we reduce ammonia loss. We may be decreasing the fertilizer nitrogen value of that manure, but that can be made up with fertilizer nitrogen.”
Urinary nitrogen is the biggest source of reactive nitrogen on dairy farms, and a lot can be done to reduce that source. If the goal is abate ammonia and nitrous oxide emissions, those values can be monitored through MUN (milk urea nitrogen). “The urea concentration in the milk reflects the urea concentration in the urine,” said Powell. “Most dairy farmers have access to (herd) MUN from their milk coop, and some farmers get MUN for individual cows.”
Powell explains urea is circulated throughout the cow to achieve equilibrium between urea in the blood, urea in the milk, and urea in the urine. Excess feed nitrogen increases urea in the system, and the pathway for eliminating that excess nitrogen is through the urea in the urine. “Urea is in equilibrium, so MUN corresponds to urea in the urine and urea in the blood,” he said. “The urease enzymes which are in feces and in the soil hydrolyze that urea very quickly to ammonium, and through various processes, this is lost as ammonia. In the soil system, the ammonia goes into nitrate, and the nitrate denitrifies and becomes a source of nitrous oxide.”
Research has shown that tannins, such as those found in birdsfoot trefoil, decrease urinary nitrogen because tannins bind feed protein. Proteins bypass the rumen and are used more efficiently in the hindgut. The protein in alfalfa is not used efficiently by the cow, and most of the protein ends up in feces or urine. Research is aimed at making more efficient use of the nitrogen in alfalfa by increasing the tannin content.
“As we feed tannins,” said Powell, “we reduce urinary nitrogen loss and get more nitrogen in feces where it can be more efficiently recycled.”