Current Progressive Dairy digital edition

How environment affects forage digestibility

Mike Hutjens for Progressive Dairyman Published on 05 May 2017
Corn field

Forages continue to be the “backbone” of successful dairy rations. Crude protein used to be the “focus” nutrient, but the focus has shifted to fiber digestibility for several reasons: digestible neutral detergent fiber (NDF) impacts microbial protein production, feed intake, rumen volatile fatty acid profile and production, and rumen pH associated with animal health.

Seed companies have responded by developing low-lignin alfalfa, BMR corn silage and sorghum silage, and hybrids selected for higher fiber digestibility.



Forage testing labs have contributed by developing fiber tests including NDF digestibility at 24, 30 and 48 hours after in vitro fermentation, undigestible NDF at 30, 48, 120 and 240 hours after in vitro fermentation, and total tract NDF digestibility. Dairy farmers use these tools to build rations containing 55 to 75 percent of the total ration dry matter in their feeding programs.

A number of factors challenge nutritionists, veterinarians and dairy farmers to improve or maintain forage digestibility. These factors can be grouped in controllable and uncontrollable factors.

Readers may disagree with the grouping of these factors, but each one can impact forage digestibility. Feel free to add to and modify the list. You may want to grade each factor on your farm. (An A means it is a positive factor, while a C means room for improvement, and an F means it cannot be changed or improved.) Table 1 (page 95) has an illustration from our Wisconsin dairy farm when I was in college.

Grading the Hutjens Dairy Farm near Green Bay, WI

Uncontrollable factors

Challenge: Soil fertility reflects the availability of soil nutrients for plant growth. Nitrogen, phosphorus and potassium levels for forage crops must be optimal. Some forages may need other macro- and microminerals. Your soil specialist can provide optimal levels for your crop with target yield.


Besides fertility considerations, soil type can be a factor, such as sandy, clay or black loam. These soil types have variable impact on water and nutrient retention, soil compaction, organic matter content and nutrient release.

Solution: None, unless you have optimal soil type or are willing to move to the Corn Belt states; however, manure can be a plus to improve soil characteristics, organic matter and nutrients.

Challenge: Growing conditions are out of dairy farmer control unless you have irrigation. Three areas of growing conditions are listed below:

  • Sunlight is needed for plant photosynthesis of nutrients. The level of light can shift fiber digestibility and level. Legumes raised at higher elevations can be higher in digestibility due to favorable growing conditions.

Solution: Raise forages in areas with more sunlight or adjust plant density by changing seeding rate and plant numbers.

  • Temperature is important as growing degree days impact growth, yield and quality. Grasses and legumes raised under cooler conditions are higher in fiber digestibility (for example, first-cutting legumes and grasses in the Midwest region).

    Grasses can be classified as warm-season or cool-season, with cool-season grasses higher in digestible fiber compared to warm-season or tropical grasses. The optimal temperature for cool-season grasses is 68ºF. For warm-season grasses, the optimal temperature is 86ºF.

    For each 1-degree increase on the Celsius scale in optimal temperature (or each 1.8-degree increase on the Fahrenheit scale), forage digestibility is lowered by 0.3 to 0.7 percent. High temperatures can promote lignin formation.

Solution: Select forages adapted for your area based on historical data.

  • Water continues to be a wild card every year. Wet, cool spring conditions delay planting of annual forage crops and can delay harvest, increasing plant maturity. Heat stress and drought typically increase fiber digestibility (the good news) but reduce yield (the bad news).

    Drought can also lead to limited to no forage production. Corn silage under drought conditions can lead to shorter plant height (especially during the vegetative phase), while grain production is reduced if pollination occurs under dry conditions.

Solution: Irrigation if it is available and feasible.


Challenge: Frost damage is important when forages need more time to mature, especially corn silage. A killing frost stops leaf and plant growth, leading to loss of leaf and plant dry matter. Delaying harvest for several days allows for the corn silage to dry down to optimal dry matter levels for favorable silage fermentation and avoids excess leaching.

Frost damage can lead to higher levels of NDF with higher fiber digestibility based on stage of maturity (for example, if the corn plant was in a vegetative stage when frost damage occurred). With legume forages, if 45 to 50 days of regrowth has occurred, another harvest can be taken with minimal risk to spring regrowth or winterkill.

Solution: Select hybrids with the optimal growing degree days for your area. If the corn silage is excessively wet, adding 50 pounds of a dry byproduct may increase dry matter content of the silage by 1.5 percent units.

Challenge: Fungal and disease damage reduces plant growth and yield, which can lower fiber digestibility as the plant is compromised. Wet growing conditions favor fungal growth.

Solution: Strategic application of fungicides can protect the plant and improve yield and quality. Forage crops can be selected that have fungal or disease protection.

Challenge: Weeds rob moisture from the forage crop, may lead to premature harvest to prevent weeds going to seed, and weed plants generally are mature with lower levels of digestible fiber. Forage digestibility generally will be lower due to weed contamination.

Solution: Forage crops can be genetically modified, allowing application of herbicides to control weed growth.

Controllable factors

Challenge: Leaf-to-stem ratio reflects quality in legume and grass forage production. Maturity has an impact as leaf dry matter contains more nutrients, and higher digestibility declines while stem dry matter content will be higher in fiber. High temperatures reduce the leaf-to-stem ratio.

Solution: Harvest at optimal maturity and avoid harvest losses. Select hybrids with great leaf dry matter.

Challenge: Grain-to-stalk ratio applies to corn silage and other cereal silages. As the level of starch (grain) increases, fiber digestibility and levels declines. The good news is: The extra nutrients formed in the grain portion compensates for decline in fiber digestibility.

Solution: Processing corn silage may be able to improve fiber digestibility by opening the corn plant stem to expose fiber to bacterial fermentation with greater surface area. Select hybrids bred for higher fiber digestibility in the stalk and plant material.

Challenge: Stage of maturity is the largest controllable factor dairy farmers determine. As plants continue to mature, the stem or stalk becomes more lignified, which reduces NDF digestibility and can prevent or block fermentation and digestion.

Solution: Dairy farmers must harvest forage at optimal maturity, have adequate equipment and labor, or hire a custom forage harvesting company who can complete a timely harvest at optimal maturity.

Challenge: Harvest losses can significantly lower fiber digestibility as leaf loss during the harvest of legumes shifts the leaf-to-stem ratio with a higher proportion of stem dry matter lower in digestibility.

Solution: Select harvesting equipment and systems that minimize leaf loss (haylage versus hay systems, windrow merger, wrapped baleage and baling hay with moisture to reduce losses).

Challenge: Storage losses can have lower fiber digestibility risks in silage systems. Fermentation losses reflect the conversion of sugars and starch to heat and oxidation if the fermentation time is extended, increasing fiber concentration. Poor packing leads to loss of dry matter and lower feed value. Surface spoilage can represent 3 to 6 percent dry matter loss.

Solution: Use of a research-based silage inoculant can reduce dry matter losses by 2 to 4 percent and increase energy content by 2 to 3 percent. Silage inoculants may introduce enzymes, improving fiber digestion. An oxygen barrier covering should be considered.

Challenge: Yeast and mold growth can lower energy value of silage due to organisms using sugar, starch or digestible fiber. If yeast counts exceed 100,000 colony-forming units per gram of ensiled feed, silage stability at feedout can be compromised. Field molds and spores can enter the plant through plant damage such as insect injury, down the silk channels during pollination or through the root system. Storage molds grow in anaerobic conditions and can produce mycotoxin.

Solution: Use silage inoculants that produce acetic acid to reduce or slow yeast growth at feedout. Yeast reduces lactic acid levels, allowing mold to grow. Healthy plants can reduce mold growth along with fungicide application. Select forage hybrids with desirable genetic traits to improve plant health.  end mark

PHOTO: Staff photo.

Mike Hutjens
  • Mike Hutjens

  • Professor Emeritus of Animal Sciences
  • University of Illinois – Urbana
  • Email Mike Hutjens