Several months ago, my friend Corey called me with a cattle feeding question. He oversees a small biodynamic dairy in Columbia County, NY. He had just gotten the bulk tank results back from the lab. They told him that milk urea nitrogen (MUN) was running 25 milligrams per deciliter (mg/dl).
MUN is a useful tool when looking at a dairy ration and possible causes of low milk production and unsatisfactory fertility. Urea level is an indicator of the biochemical balance in the rumen. Thus, excessive MUN levels (greater than 18 mg/dl) signal a red flag in the protein imbalance, a shortage of carbohydrates or poor microbial environment in the rumen. When he called me, Corey 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 MUN, but I warned that the energy level should be adequate, meaning that the hay should be early cut.
Here’s what happens with excess protein in a ruminant’s diet. According to Virginia Ishler, recently retired Penn State dairy specialist, cows break down excess protein into ammonia. The liver converts ammonia into urea, which ultimately is voided in urine. She said that excretion of excess urea requires energy, since cows have to expend at least two megacalories of energy to excrete the excess urea into the urine. This cost is estimated to be equivalent to 1.1 lbs. of milk per unit increase in MUN.
According to Ishler, excess concentrations of urea in the blood can affect milk production, reproductive efficiency, embryo survivability, immune function and the environment outside the cow. Research has shown that a strong positive correlation exists between blood urea nitrogen (BUN) and MUN. Management-wise, this correlation is quite beneficial: it’s a lot easier (as far as the cow is concerned) for humans to assay something in her milk rather than in her blood.
Not all nitrogen compounds in a ruminant’s system are proteins, which can be quite complex. Expressed simply as possible, a protein consists of multiple peptides, each of which consists of multiple amino acids. Other simpler nitrogenous compounds are called non-protein nitrogen (NPN). This grouping includes urea, ammonia and simple amino acids. In the rumen these simpler compounds can be coupled with carbohydrates, a class including fibers that humans can’t digest. These NPNs and carbs are crafted by specialized 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, about 25 years ago, involved a Central New York farmer named Carl who had one of the highest Holstein herd averages in his county. As a salesman for a national mineral company, I handled his feeding program. That consisted of mixed mostly grass haylage, high moisture shelled corn (HMSC), a slice of dry hay, soy 48%, minerals and salt. Normally haylage protein tested 16% dry matter (DM) basis, of which 45% was soluble; haylage moisture was 60%.
His HMSC tested 10% protein, of which 40% was soluble. Protein in his soy was about 35% soluble. The approximately 5 lbs. of grassy hay was about 10% protein, 20% of which was soluble. Typically, his cows averaged about 70 lbs. 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 protein solubility under 35%. We were “skating on thin ice.”
The next year he got into trouble nutritionally and cow health-wise. That growing season, a really wet one, yielded haylage that still tested 16% protein, but its moisture was 65%; its protein solubility was 55% protein solubility increased with moisture. We realized that, figuratively, we had “gone through the ice” when three fall-fresh cows, each giving 100 lbs. of milk, went down, never getting up. Carl asked 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 and molasses. These carbs could then be coupled with excess NPN to be made into protein by rumen microbes.
Expressed simply, the excess NPN – absent the carbs – meant that 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 levels. Ammonia is a strong base, hence the elevated pH.
Here’s how we fixed this problem: We replaced about two-thirds of the soy protein with protein from distillers dried grains (DDG). DDG runs about 30% total protein, about 10% of which is soluble. I got Carl to feed a little more dry hay as well as some sodium bicarbonate. Total dietary protein solubility dropped back down to 40% – still quite a stretch from the 35% maximum that we need to honor. But Carl’s cow problem was gone and he remained my customer.
In 2007, another CNY dairyman wanted me to figure out why recently bred cows were “repeating.” He had run out of corn silage and took up the dietary slack with nice hay. His Dairy One MUN score was about 22. Most dairy researchers recommend a MUN range of 12 – 15. It was safe to presume, based on the high MUN value, that his cows had high blood ammonia, which can cause embryonic mortality.
I had him buy some molasses; he began feeding 2 lbs./head/day. Rumen microbes coupled the new source of carbs with the excess NPN to make usable proteins. The bred cows remained pregnant, freshening on schedule.
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