Cow diet affects milk fat

by Deborah Jeanne Sergeant

Want more milk fat? Improve your herd’s rations, said Tom Jenkins, DVM, Ph.D., with Clemson University. He presented “Feeding Fat in Lactation Rations” as a recent webinar hosted by Agricultural Modeling and Training Systems.

“A multitude of factors affect milk fat content,” Jenkins said. They include nutritional factors and non-nutritional factors. “Both are very important.”

Non-nutritional factors include genetics, stage of lactation, season, parity, breed and disease.

“The first 90 days, milk fat will be very high because of maternal fat stores,” Jenkins said. He also noted that most cows experience a “summer slump” for milk fat and protein.

But operators can control the nutritional factors and use them to influence milk fat and mitigate the effects of the season or stage of lactation.

The nutritional effects on milk fat include high starch, low rumen pH, wild yeast, feed delivery (TMR mixing, feeding frequency and push-up), feed facilities (bunk space and overcrowding), ionophores and high fat. Identifying what’s lacking “can be very frustrating,” Jenkins said. “These are not individual events and they all interact with each other.”

Since more than one factor can interact with another, dairymen trying to figure out what is lowering their herd’s milkfat may struggle to do so for a while.

“That can make it very, very difficult at times to improve milkfat,” Jenkins said.

He explained that milk fat is a collection of fatty acids and that rumen plays a central role in the synthesis of fat.

“If the mammary gland is the engine driving the synthesis of milk fat, I see the rumen as the gas pedal and the brake,” Jenkins said.

If farmers can increase the grams of acetate in lactating dairy cows, they can increase their production of milk fat. “Supplying more acetate is stepping on the gas,” Jenkins said.

Conjugated linoleic acid (CLA) offers numerous health benefits to cow’s rumen. “Just a few grams of CLA cause dramatic – 20 to 25% – increases of milk fat yield,” Jenkins said.

He likens feeding decisions to a crossroads.

“Trans 11 pathway CLA rumencial acid won’t cause problems with milk fat or trans 10 pathway, a known inhibitor of milk fat,” Jenkins said.

One example is rumen unsaturated fatty acid load (RUFAL). Soybean oil and corn silage-based TMR are “big contributors to RUFAL,” Jenkins said. “Hay and haylage will increase linoleic acid.”

He said microorganisms respond the same to unsaturated fat whether from additives or the primary diet. He added that the root cause of what causes linoleic acid is too much RUFAL. “Everything you put in front of a cow contributes RUFAL to the rumen environment,” he said.

He explained this is important because high RUFAL disrupts the microbial population. These effects happen quickly and are enhanced by interactions among various nutritional risks.

“In a very, very short time, the microorganisms are bothered by the presence of linoleic acid,” Jenkins said. “If you go too high on RUFAL, the microorganisms in the rumen will respond very quickly.”

But microorganisms respond to linoleic acids differently.

“Saturated fatty acids are not the problem,” Jenkins said. “We have to limit the amount of RUFAL we use,” Jenkins said. “All RUFAL isn’t created equal.”

Low-risk RUFAL sources include hay, unprocessed corn, unprocessed cotton seed, whole oilseeds and Ca-salts of fatty acids. High-risk RUFAL sources include process corn, ground oilseeds, fats/oils and DDG.

“I don’t try to tell customers how much RUFAL is too much,” Jenkins said. “The amount that becomes a problem varies.”

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