Nitrogenous NuisanceLast Saturday my friend Brian called me with a feeding management question. He manages a small biodynamic dairy in Columbia Co., NY. He had just gotten the milk tank sample results back from the lab; from those he learned that milk urea nitrogen (MUN) was running mg/dl. MUN is a useful tool when looking at a dairy ration and possible causes of low milk production and fertility. Urea is an indicator of the balance of the rumen system, so excessive levels of MUN (greater than 18 mg/dl) signal a red flag in a protein imbalance, a shortage of carbohydrates or poor microbial environment in the rumen. When he called me, Brian was awaiting forage test results on some high quality, mostly alfalfa baleage.

He asked me if he should replace this baleage with some grassier hay. I said yes; that would lower the protein level and thus the MUNs, but I warned that the energy level should be adequate, meaning the hay should be early cut.

Here’s what happens with excess protein in a ruminant’s diet. Dr. Doo-Hong Min, Extension forage specialist at Michigan State University, explained it this way: Cows break down excess protein into ammonia. The liver converts ammonia into urea, which ultimately is voided in urine. Quoting Min directly, “Excretion of excess urea is an energy requiring process, since cows have to expend two megacalories or more of energy to excrete the excess urea through the urine. This cost is estimated to be equivalent to 0.5 kg (1.1 lbs.) of milk per unit increase in milk urea nitrogen. Excess concentrations of urea in the blood can affect milk production, reproductive efficiency, embryo survivability, immune function and the environment. It has been shown that there is a strong positive correlation between blood urea nitrogen and milk urea nitrogen.”

Not all nitrogen compounds in a ruminant’s system are proteins. Proteins are complex. Expressed as simply as possible, a protein consists of multiple peptides; each peptide consists of multiple amino acids. Other simpler nitrogenous compounds are called non-protein-nitrogen (NPN), which typically include urea, biuret, ammonia and simple amino acids. In the rumen, these simpler compounds can be coupled with carbohydrates (including humanely indigestible fibers), ultimately crafted by microbes into proteins.

Following are two cases where excess NPN caused bovine nutritional train wrecks. Neither case involved human addition of urea, ammonia or any other form of synthetic nitrogen.

The first case involved a Mohawk Valley farmer named Carl. He had once of the highest Holstein herd averages in his county. As a sales rep for a national mineral company, I handled his feeding program, which consisted of conventional haylage, high moisture shelled corn (HMSC), a slice of dry hay, soy 48%, minerals and salt. Normally his haylage tested 16% protein (dry matter basis), of which 45% was soluble; haylage moisture was 60%. His HMSC tested 10% DM protein, of which 40% was soluble. The protein in the soy was about 35% soluble. The approximately five pounds of grassy hay was about 10% protein, of which 20% was soluble. Typically, Carl’s cows averaged about 70 pounds of 3.5% test milk.

This feed program averaged about 17% total DM protein, of which about 40% was soluble. In cattle nutrition, we strive to keep total protein solubility under 35%. I knew we were skating on thin ice. The next year we got in trouble nutritionally (and cow health-wise). That next growing season – a really wet one – yielded haylage that still tested 16% protein. But its moisture was 65%, and its protein solubility was 55% (protein solubility tends to increase with moisture). We realized that, figuratively, we’d “gone through the ice.” Three fall-fresh cows, each giving about 100 pounds of milk, went down … and never got up. Carl told me to make the problem go away.

Here’s what had gone wrong. The extreme elevation in total dietary soluble protein (now up to 45%) meant that the rumen was bombarded with huge amounts of NPN – primarily cow-made urea and ammonia – and amino acids that rapidly degraded to ammonia. The cows needed more dietary energy (like non-structured carbohydrates in corn meal or molasses), which could then be coupled with excess NPN to be made into proteins by rumen microbes.

Expressed simply, the excess NPN, absent the carbs, meant more ammonia was available to escape through the rumen wall into the cow’s circulatory system. The overtaxed liver couldn’t convert the excess ammonia to urea and uric acid. Surplus ammonia remained in her circulatory system, elevating her serum pH to dangerously high (and fatal) levels. Ammonia is a strong base.

How did we fix this problem? I replaced about two-thirds of the soy protein with protein from distillers dried grains (DDG). DDG runs about 30% total protein, of which only about 10% is soluble. I also got him to feed a little more dry hay, as well as some bicarb. Total dietary protein solubility dropped back down to 40% – still quite a stretch from the 35% maximum we need to honor. But Carl’s cow problem was gone, and he remained my customer.

Fast forward to summer 2007. Paul, in Otsego Co., wanted me to figure out why recently bred cows were “repeating.” He had run out of corn silage and taken up the dietary slack with some nice hay. He was on test, so I asked him what his herd MUN readings were. His MUN score was about 22. Some dairy researchers recommend a range of 10 to 14, while others recommend a range of 10 to 12. It was safe to presume, based on the high MUN value, that Paul’s cows had high blood ammonia. Elevated blood ammonia can cause embryonic mortality. I had him buy in some molasses and begin feeding two pounds of the sticky stuff per head per day. Rumen microbes coupled the new source of carbs with the excess NPN to make usable proteins. There were no more “repeats.”