Monitoring nutrition in the pasture

By Thomas E. Noyes

I have had numerous dairy graziers tell me their cows did not milk well this summer.
“Why didn’t my cows milk as well this summer, and how could I have supplemented them?” The answers are not simple, but I have some suggestions.
A challenge. The feeding of dairy cows in confinement systems is done with very good precision. Forages fed are analyzed; computer programs are used to balance diets; mixer wagons with scales can accurately blend feed ingredients into a total mixed ration (TMR) that’s fed to the cows; and we can monitor what is consumed by weighing and analyzing the refusal.
Cows in a grazing system present more of a challenge. However, there are some tools and procedures that you can use to determine intake and monitor the diet balance.
A simple tool would be the monitoring of the milk urea nitrogen in the bulk tank milk. Many dairy labs can run this test, including DHI.
The milk’s urea nitrogen content is related to blood urea nitrogen.
As cows consume feed, the proteins are broken down by the microorganisms in the rumen to ammonia, then converted to microorganism protein.
If there is excess protein in the diet, or if there is not enough energy for the rumen “bugs” to utilize all the protein, then some of the ammonia will pass through the rumen to the liver and be converted to urea.
This will elevate the blood urea level and when the blood urea level is elevated, then the milk urea nitrogen becomes elevated.
Calculations. When the lab measures milk urea nitrogen, the expected normal range is 10-14 mg/dl (milligrams per deciliter). Consistent levels more than 16 mg/dl are considered high, and may be caused by too much protein, or not enough energy to utilize the protein.
Low milk urea nitrogen consistently under 10 mg/dl means too low of a protein level or too much energy for the protein level in the diet (watch for acidosis).
Dry matter. A more difficult measure would be to determine the dry matter intake of both the supplementation and the pasture.
The supplementation dry matter is quite simple to measure. Just weigh dry matter before feeding, and subtract out any refusals.
Measuring. For the pasture dry matter intake, you first measure the dry matter in a paddock before the cows are turned in (use a pasture stick, rising plate meter or do some actual clipping), then measure the dry matter after they come out of the paddock.
The difference gives you dry matter consumed.
Divide the total dry matter consumed by the number of cows and you have the dry matter intake per cow.
To make this more accurate, you need to determine the dry matter of the various feeds that are being fed, especially the dry matter of the pastures and any silages being fed.
Once you have determined dry matter intake, and if you have had feeds analyzed, you could have your nutritionist do a computer check on the balance of the diet.
Some of the computer programs will calculate expected dry matter intakes according to the feeds being fed.
This will show you what nutrients are lacking or if there is an imbalance of the protein and energy.
Talk to the cows. Finally, let’s not forget what the cows are telling you.
What are the body conditions of the cows? Thin cows, especially in early lactation will mean energy shortage.
How high the peak milk production was, and how soon after freshening (you may need DHI reports to determine this) is another energy intake-related factor.
Observe the cows while they are grazing, as this can tell you a lot about the palatability of the pastures.
Exact science? Troubleshooting nutrition for grazing dairy cows is more difficult.
There probably will not be as exact a way like a total mixed ration; however, some of the tools that I have suggested could help you evaluate some possible causes for poor performance.
(Thomas E. Noyes is the Wayne County Extension dairy agent. Questions or comments can be sent in care of Farm and Dairy, P.O. Box 38, Salem OH 44460.)