What's Behind this Summer's Low Butterfat Tests?

Posted: August 12, 2008

For many producers across the state, it has been a struggle to maintain fat tests above a 3.4%. This has occurred on non-grazing herds and has been fairly consistent throughout the summer, regardless of temperature. This seems to be occurring more on herds feeding a high forage based ration and on an average milk production between 70 and 80 pounds. The one commonality is these herds are feeding corn silage. What are some issues that could be challenging butter fat percent?

Holstein Eating Hay in Tiestall Barn

Last year many parts of the state experienced some drought conditions. Even though it may not have been as severe as years past, the corn tended to be shorter than normal with very good ear development. This resulted in many corn silage samples testing relatively low in neutral detergent fiber level (NDF; <40%) and high in starch level (>37%) This coupled with a storage time of over 8 months has resulted in a starch fermentability that is probably very high. Many nutritionists are formulating rations with total NDF contents between 30 and 32% which is fine but now the majority of that NDF is coming from corn silage. The effectiveness of the NDF of the corn silage may not be adequate and not providing enough “chew factor” for cows to maintain good rumen health. This is confounded by the fact many producers over the years have gradually gone to a heavier corn silage based ration. The end result is that even with the total pounds of NDF being adequate and meeting recommendations, herds can still observe a milk fat depression.

If the cows are healthy (not showing signs of acidosis) and are not losing body condition, then it may be worth waiting it out.

As diets contain more dry matter from corn silage, nutritionists are adding dry forages such as straw or grass hay in an attempt to bring other effective fiber sources into the ration. Another challenge with feeding higher corn silage based rations is that sorting can become a greater issue if these dry fiber sources are not adequately chopped to a particle size to prevent sorting. In addition, if the corn silage is drier than recommended it will be even more difficult to get a good mix with other dry ingredients. Therefore combined with potentially more reliance of fiber from corn silage as effective fiber and potential for greater sorting, it is not surprising there are some lower butter fat tests being observed in herds this year.

If the corn silage starch level is high, evaluate the cereal grain sources. If finely ground corn grain is being fed (i.e. > 80% of the corn going through a 16 mesh screen), then moving to a coarser grind (only 60% going through a 16 mesh screen) may be prudent. If high moisture corn is being fed, incorporating some soluble fiber sources such as soy hulls could help to reduce the amount of corn grain fed.

Incorporating more grass or alfalfa silage to replace corn silage dry matter may help off-set the low NDF/high starch diet. This change would require re-formulating the diet for protein and protein fractions.

Another concern relates to dietary fat. If the ration being fed contains corn grain, corn silage, distiller’s grains and no other carbohydrate sources or soluble fiber sources, the ration may be limited in potential effective fiber. The starch fermentability may be high that results in lower biohydrogenation of fat in the diet. This has been shown to contribute to reduced milk butter fat.

With incorporation of the aforementioned strategies, it is possible that fat test will improve only slightly. If corn silage makes up over 35 to 40% of the ration dry matter, then components may not turn around until 2008’s corn silage begins. The expectation is that nutritionists can work miracles to correct nutritionally related problems. However, sometimes there are circumstances outside their control and when evaluating the labor and expense involved in some changes to correct the problem, maintaining a 3.4% milk fat may be okay compared to what it may cost to increase it .20-.30%. If the cows are healthy (not showing signs of acidosis) and are not losing body condition, then it may be worth waiting it out. The escalating costs with producing milk may require a different interpretation of “is a problem really a problem”.


Virginia Ishler, Extension Associate
Gabriella Varga Professor
Penn State Dairy Alliance, a Penn State Cooperative Extension Initiative