The author is president of the William H. Miner Agricultural Research Institute, Chazy, N.Y.

Carbohydrates often make up 60% or more of the ration dry matter and provide 70% to 80% of the cow’s energy needs. Achieving the right balance between fiber and fermentable carbohydrates, such as starch, optimizes salivary buffering and rumen acid production. This combination promotes good rumen health. Optimal rumen pH is central to ruminant health and productivity — and, of course, cows are ruminants.

Low rumen pH is often associated with reduced de novo synthesis of fatty acids in the mammary gland. These short-chain fatty acids, produced from scratch in the udder, use building blocks of acetate and butyrate from fiber fermentation in the rumen. So, getting the ration content of fiber and starch right to optimize rumen pH and fiber fermentation is critical.

The interaction between dietary fiber and starch has been evaluated in many studies over the years. In particular, research has focused on how feeding too much starch reduces rumen fiber fermentation and energy available to the cow.

Work at Nebraska found that pure corn starch with a high rate of in vitro starch digestion reduced neutral detergent fiber (NDF) digestion of alfalfa by about 42%, as shown in Table 1. In contrast, ground corn with a slower starch digestion rate only reduced NDF digestion by 17%. Ground sorghum, with the slowest rate of starch digestion, had little effect on NDF fermentation. Similar results were noted when these starch sources were combined with other common NDF sources such as corn silage or bromegrass.

How severe the negative effect of starch is on rumen NDF digestion is a function of its digestibility and, of course, how much starch is in the ration. The product of starch content and its rumen digestibility is commonly called rumen fermentable starch. We know that excessive rumen fermentable starch will seriously reduce rumen NDF fermentation.

At the Miner Institute, we have been focusing on lactation response to dietary undegradable NDF at 240 hours of fermentation (uNDF240) and physically effective NDF (peNDF). We have been using the term physically effective uNDF240 (peuNDF240). This measure is the uNDF240 content multiplied by the percentage of ration or forage particles retained on the 4 mm sieve of the Penn State Particle Separator when measured on farm. In the lab, it would be the fraction of dried particles retained on the 1.18 mm screen, which is the official laboratory method for determining peNDF as defined by Dave Mertens in 1997.

We combined data from seven separate studies and have been able to assess how cows respond to a fairly wide range of dietary uNDF240 and peuNDF240. For typical corn silage and hay crop silage-based rations, dry matter intake begins to be limited by rumen fill when uNDF240 reaches about 10% of ration dry matter. With these higher forage and lower fiber digestibility diets, the cow’s ability to turn over fiber in the rumen slows down and reduces intake.

On the other hand, with lower forage diets, as they approach about 7% uNDF240, the risk of subacute acidosis climbs. Consequently, nutritionists often aim for somewhere between 8% and 9.5% uNDF240 in the ration dry matter for corn silage-based rations.

Fiber, starch, and milkfat

Our most recent research has focused on the interplay between rumen fermentable starch and uNDF240 at the lower end of the fiber range commonly found in milk cow rations. We have focused on diets with about 7% uNDF240 or about 4% peuNDF240. In this recent work, we directly measured the uNDF240 content of the ration particles that were retained on the 1.18 mm screen, and so these numbers differed from those in the data base.

In this study we assessed two levels of rumen fermentable starch fed in diets with either lower or higher peuNDF240, though all diets were on the lower side of uNDF240. For example, the levels were about 7% of ration DM.

The lower rumen fermentable starch diets contained 20.7% starch and 16.8% rumen fermentable starch, and the higher starch diets contained 24.7% starch and 19.1% rumen fermentable starch. The dietary starch was manipulated by simply feeding more or less finely ground corn meal.

A key to appreciating the results of this research is that even the highest level of rumen fermentable starch that we fed (19.1% of ration dry matter) is not that high compared with what is commonly fed on dairy farms. Often, diets for high-producing cows contain 22% to 23% rumen fermentable starch.

Despite the moderate level of rumen fermentable starch, it was sufficient to reduce milkfat and 3.5% fat-corrected milk compared with the 16.8% diets as shown in Table 2. Also, cows fed these moderately high rumen fermentable starch diets tended to have lower efficiency of fat-corrected milk production. Reflecting the milkfat depression, these cows also had lower rumen acetate-to-propionate ratios.

The bottom line is that feeding rations with moderately high rumen fermentable starch reduced 3.5% fat-corrected milk production when diets had a relatively low content of uNDF240 and peuNDF240. Undegradable NDF240 has become a common measure of ration fiber characteristics, and understanding how cows respond to starch at varying levels of uNDF240 is necessary.

Our research indicates that the risk of milkfat depression increases when the dietary uNDF240 to rumen fermentable starch ratio is 0.36 or lower, or when the peuNDF240 to rumen fermentable starch ratio is 0.21 or lower. Although more research is needed, monitoring these ratios may help to avoid milkfat-depressing diets.

Cows are sensitive to starch

The negative interaction between too much starch and rumen fiber digestion is a well-known story. But recent advances in measuring fiber and starch characteristics allow us to better assess feeds and formulate rations.

We need to understand the interaction between rumen fermentable starch and uNDF240 or peuNDF240 when formulating rations. High-producing cows with high feed intake are sensitive to starch and its rumen fermentability. When formulating and feeding lower uNDF240 rations, even moderate rumen fermentable starch may reduce milkfat.