With profit margins currently extremely tight, it's vital for dairy producers to be as efficient as possible when it comes to getting the most production from their herds.

Feeding cows more profitably was the focus of the opening session of the 2016 Cow College Jan. 5 at the Fox Valley Technical College Regional Center. This is the 54th year for the three-session event sponsored by Fox Valley Technical College and UW-Extension in Waupaca, Shawano and Outagamie counties.

Dr. David Combs, UW-Madison dairy nutrition specialist, was on hand to update the more than 50 producers attending on a new and better method for assessing the quality of their forage.

Combs began his presentation by stressing the importance of fiber in the diets of dairy cattle.

'In high producing dairy cows, about 20 to 25 percent of the energy for milk production comes from digested fiber in the ration,' he said. 'Balancing rations for carbohydrates — starch and neutral detergent fiber (NDF) — is critical for the health and production in high producing cows.'

Combs noted that properly assessing fiber digestion is often challenging.

'NDF is a forage test that measures the total amount of fiber in a feed, but it represents a 'bulky', slow-to-digest feed component, that can restrict feed intake and milk production,' he remarked. 'Forages are tested for NDF and lactating dairy cow diets are typically formulated to contain 28-35 percent NDF.'

He also pointed out that fiber digestibility varies significantly in different forages.

'NDF digestibility in a forage like alfalfa or corn silage will vary from around 20 percent to over 60 percent,' Combs said. 'In a dairy ration containing about 35percent corn silage, a doubling of fiber digestibility would increase the digestible energy enough to support up to 7 pounds more milk per day. Fiber digestibility is affected by growing conditions, plant genetics and forage maturity.'

New digestibility test

A new in vitro lab assay has been developed by Combs and other UW-Madison dairy scientists that predicts total tract NDF digestion (TTNDFD) in ruminants.

'This has the potential to be a real game changer,' said Greg Blonde, Waupaca County UW-Extension agriculture agent.

The in vitro TTNDFD assay predicts NDF digestion of alfalfa, corn silage, grass forages and byproduct feeds. UW-Madison researchers have validated the accuracy of the in vitro TTNDFD test against directly measured NDF digestibility in lactating dairy cattle. The University of Wisconsin recently was awarded a patent for the test and Rock River Labs in Watertown is a licensed provider of the TTNDFD assay.

Combs explained that the TTNDFD assay is different from other fiber quality measures because it is a direct quantitative predictor of fiber digestion.

'Other tests such as NDFD30, relative forage quality (RFQ), milk per acre or milk per ton (Milk 2006) are used to compare the relative differences in forage quality among alfalfa varieties or corn silage hybrids.' he said. 'These indexes can't be used to compare forages across types however, which limits their value as decision-making tools for optimizing the combination of corn silage, alfalfa and grass in dairy forage systems. These indexes also cannot be used in ration balancing.'

The TTNDFD test is designed to predict how the process of forage fiber digestion is expected to occur in high producing dairy cows.

'Think of forage quality as how far you can travel on a tank of gas,' Combs said. 'You can't calculate how far you can go unless you know how much fuel is in the tank (pdNDF), and the miles traveled per gallon (kd). How much milk your forage will make depends on the amount of potentially digestible fiber and the rate of fiber digestion.'

How it works

Combs says there are four critical factors that affect fiber digestion and the TTNDFD test accounts for each factor:

The proportion of feed fiber that is potentially digestible. Forage NDF consists of two components, a potentially digestible (pdNDF) component and an indigestible NDF (iNDF) component. The proportion of NDF that can potentially digest varies due to feed type and growing environment. On average, about 60 to 65 percent the NDF in alfalfa is potentially digestible.

The rate of digestion of potentially digestible fiber (kd). The rate of fiber digestion also differs due to forage type and growing environment. The TTNDFD values for alfalfa and corn silage are similar, but the process of NDF digestion is quite different between these two forages. In corn silage, there is a larger fraction of digestible fiber that digests slowly. In alfalfa, there is a smaller proportion of digestible fiber, but the faster rate of digestion of the potentially digestible fraction compensates for the bigger pool of iNDF.

· The rate of passage of potentially digestible NDF through the cow (kp). Both cow size and feed intake affect the passage rates of pdNDF and iNDF. As intake goes up, the rate of passage of both fractions increase, and as a result NDF digestibility declines. Passage of the pdNDF fraction is also slower than passage of the iNDF fraction so the TTNDFD test specifically measures the rate of digestion of the potentially digestible NDF.

· Ruminal and hindgut fiber digestion. Approximately 90-95 percent of fiber digestion occurs in the rumen, but digestion beyond the rumen must be accounted for if one is to accurately predict the amount of energy derived from NDF. When both ruminal and hindgut digestion are accounted for, a total-tract NDF digestion (TTNDFD) measurement can be calculated and this digestion coefficient can be directly validated with dairy cattle.

'The TTNDFD assay integrates all four of the above factors into a single value,' stressed Combs. 'TTNDFD predicts the proportion of the total NDF of the diet that will digest between the mouth and feces. The NDF that disappears between the mouth and feces is the energy from fiber that can be used to support milk production.'

According to Combs, the cost of a TTNDFD report is $26, compared to $22 for a standard analysis without TTNDFD.

Balancing the ration

Combs suggests starting with the forages and then building the TMR. 'Read forage analyses, determine NDF and starch, protein and ash content. Evaluate digestibility with TTNDFD and starchD.'

Combs stressed that TTNDFD values are consistent across feed types, and advises producers to target rations for at least 42 percent TTNDFD.

'Dynamic Kd and iNDF are compatible with AMTS and CNPES ration software, and co-product feed tables also are available,' Combs said.

TMR performance swings can be caused by changes in carbohydrates (fiber and starch in the diet as well as silage nutrient content and digestibility. 'Fiber is always lower in energy than starch (grain),' Combs affirmed. 'A 2-3 unit drop in fiber or starch digestibility will decrease milk by .5 liters,'

In concluding his presentation, Combs stressed: 'Fiber digestibility has a big impact on milk yield, and the TTNDFD test was developed to more accurately predict fiber digestibility in high-producing cows.'

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