The author has a dairy nutrition consulting business, Paradox Nutrition LLC, in Plattsburgh N.Y.
There is no “magic bullet” available to improve milkfat yields. Instead, many factors contribute to a healthy rumen and ultimately how much milkfat a cow makes. The main milkfat depression culprit is subacute rumen acidosis (SARA) that occurs when the pH of the cow’s rumen drops below 5.8. Issues occurring outside of the cow’s rumen also impact milkfat, including the effects of dietary fats and minerals.
Cow comfort and care
Anything that reduces feed access time and rumination, such as overcrowding, inadequate total mixed ration (TMR) pushups, or too much time in the holding area and milking parlor, can result in more SARA. Overstocking can raise the time the rumen faces SARA levels by two hours per day. When coupling overstocking and feed restriction together, SARA can climb up to nine hours per day, according to Miner Institute researchers. In these situations, cows slug feed, meaning they have fewer but bigger meals. Also, cows spend less time lying down and ruminating.
Heat stress can cause similar negative effects. TMR sorting can also cause SARA issues.
Starch sources have an impact
Rumen microbes ferment starch to form volatile fatty acids, with propionate being a significant end product. Excessive accumulation of propionate reduces rumen pH and encourages the growth of lactate-producing microbes. As lactate is a stronger acid than propionate, its accumulation further reduces rumen pH.
At the same level of total dietary starch, one ration containing a large amount fast fermenting starches such as barley, high-moisture corn, or bakery product may result in SARA. On the flip side, a ration containing a more slowly degradable starch like finely ground corn may not.
Fiber is a foundation
Physically effective neutral detergent fiber (peNDF) is needed to maintain rumen health. Cows need to ruminate about 8 hours per day. Saliva produced during chewing neutralizes rumen acids, elevating rumen pH so that the rumen bacteria can function well.
Physically effective fiber forms a rumen mat, which slows feed passage out of the rumen, improving digestion. Physically effective fiber also promotes the movement of rumen contents, increasing absorption of volatile fatty acids via the papillae located on the rumen wall.
Increasing fiber digestibility of farm forages allows ration grain levels and starch to be reduced, improving rumen health and reducing ration costs. But if undigested fiber (as measured by uNDF240om) is reduced too much with highly digestible forages, the risk of SARA may actually go up.
Cows need a certain amount of this very woody fiber to maintain rumen function. For example, a 1,500-pound cow would be expected to perform best when eating 5.25 to 6 pounds of undigested fiber per day.
Tracking uNDF240om intake of high-producing cow pens on your farm can be very helpful. For example, if your high-producing cows have been eating 5.5 pounds of uNDF240om for the past two months and you have been satisfied with milk production and milkfat, you will want to aim for this same amount of uNDF240om intake when forages change.
Fat has a happy medium
A large portion of diet energy should be supplied by digestible carbohydrates, but there is a limit to how much fiber a cow can consume, and too much rumen fermentable starch results in rumen acidosis. Added dietary fats are typically fed to meet the last portion of the high-producing cow’s energy needs. One pound of fat supplies approximately 2.25 times more energy than a pound of carbohydrate. Fats are not used as an energy source by the rumen microbes. Rather, fat absorption for the cow takes place in the small intestine.
The rumen microbes convert unsaturated fats to saturated fats by a process called biohydrogenation. Vegetable fats, such as those found in distillers dried grains, whole cottonseed, and whole roasted soybeans, are rumen-available unsaturated fats found in dairy cattle diets.
Up to a limit, this fat can be a great source of energy for the cow. But high levels of unsaturated fatty acids and low rumen pH can cause abnormal rumen fermentation where the microbes change their fatty acid biohydrogenation process, altering the types of fatty acid intermediates produced. When even very small amounts of these fatty acid intermediates escape from the rumen, they can cause milkfat depression.
Additional energy needed beyond that supplied from carbohydrate fermentation and rumen-available fat can be provided by rumen inert fat. A number of rumen-inert fats are available on the market. Prilled saturated hardened palm fatty acids have been found to improve milkfat percent, especially in later lactation cows.
Sugars can improve milkfat, too
Typical U.S. lactating dairy rations, with no added sugars, contain about 1.5% to 3% sugar. But 6% to 8% dietary sugar may actually be optimal. A common effect of sugar supplementation is an improvement in milkfat percentage and/or milkfat yield. There are at least three possible reasons for this.
First, sugars typically elevate rumen butyrate needed for milkfat synthesis. Second, substituting sugars for starch can help to control rumen pH. Finally, added dietary sugars have been shown to promote normal fatty acid biohydrogenation and reduce the amount of abnormal fatty acid biohydrogenation, which compromises milkfat.
Feed additives may be helpful
Although a feed additive should not be relied upon to fix all of the nutrition and management issues that may depress milkfat, a few may be helpful. Always ask for and evaluate controlled research on any additive before purchasing. Buffers such as sodium bicarbonate or sodium sesquicarbonate are generally fed at a rate of 1% to 1.5% ration dry matter (DM) for the purpose of neutralizing rumen acids.
Yeast products can improve fiber digestion and reduce rumen acidosis. Researchers in Spain found that when they added a specific live yeast to the diet, it reduced the amount of time during the day when rumen pH was below 6.0 from 9.5 hours per day down to 4.1 hours per day.
Especially during heat stress, raising the dietary cation-anion difference (DCAD) of the lactating diet can reduce metabolic acidosis and improve milkfat. Generally, about 125 grams per cow per day of potassium carbonate are needed to increase ration potassium (K) up to 1.5% to 1.7% and ration DCAD up to +30 to +40 meq DCAD per 100 grams of dry matter.
Diet consistency matters
Many dairy nutritionists evaluate daily variation in dry matter intake of cow pens or daily variation of the bulk tank. Reducing this variation is often associated with elevating milk production and milkfat.
Improvements in TMR preparation can help. This includes feeding more accurately for pounds of silage dry matter, better scale accuracy, using the best ingredient loading order, mixing speed, and mixing length, as well as improving the function of the mixing equipment. Excellent feed managers generally evaluate forage dry matters on a weekly basis and more often if a change is expected.
