Digest your digestibility

By Mike Rankin

Even if you’re just a casual reader of forage-related information, hopefully you’ve been able to grasp the concept that forage fiber digestibility is now a big deal.

To be sure, the total fiber content of forage, as measured by neutral detergent fiber or NDF, is perhaps the first and most important metric to know, but following closely behind is the digestibility of that fiber.

Livestock producers, nutritionists, agronomists, and researchers are currently exploiting these foundational forage quality concepts in a variety of ways.

Why?

Because highly digestible forages can boost dry matter intake and cut feed costs in the form of additional supplements. It also doesn’t cost much more to harvest highly digestible forage compared to a similar forage with low digestibility.

A lack of energy in the livestock diet often limits performance, especially for lactating cows or growing animals. In a forage-based diet, high energy often is dictated by fiber digestibility and the rate of digestion.

Though it’s true that protein is an important component of an animal’s diet, it rarely is the most limiting factor. It is very possible to have high-protein forage that is low in fiber digestibility. Wilting hay that is rained on often fits into this category.

How did you do?

Metrics such as NDF digestibility (NDFD), undigested NDF (uNDF240), total tract NDF digestibility (TTNDFD), relative forage quality (RFQ), or a summative TDN calculation now litter most forage test analysis sheets. If they don’t, perhaps it’s time to investigate using a different forage lab.

With the growing season heading into the home stretch, how did your forage stack up for fiber digestibility in 2018? The potential range of results is wide, as illustrated in the chart below. Grasses generally contain both more fiber but also a higher percent of digestible fiber than legumes.

Source: University of Wisconsin

Three things

As you sit down and assess the fiber digestibility status of 2018 forages, keep in mind that there are primarily three factors that will explain the results — good or bad.

1. Growing environment: This is the one factor that can’t be controlled and in 2018 will likely have a profound negative impact on fiber digestibility in regions plagued by hot weather and drought. Cool temperatures, especially at night, will have a positive impact on fiber digestibility. This is often why the highest NDFD values are seen for first cutting or in fall-harvested forage.

2. Time of harvest: Fiber digestibility declines with plant maturity. Hence, the stage of plant development when forage is cut or grazed will in most situations have the largest impact on the harvested fiber digestibility. Though grasses have potentially the highest NDF digestibility, it also declines at a faster rate than is the case for legumes. To capture the “grass advantage,” stands must be cut early as fiber digestibility declines rapidly once seedheads appear.

3. Genetics: Species and variety selection is another way to improve fiber digestibility. Among grasses, species such as meadow fescue and ryegrass have proven superior to their counterparts. Also, since the importance of fiber digestibility has risen to prominence, plant breeders have developed alfalfa varieties that are significantly better than previous offerings.

Though not a primary factor, fiber digestibility can be severely impacted when cut and wilted forage gets rained on. This occurs primarily because highly digestible carbohydrates leach from the plant tissues.

Animal performance hinges on fiber digestibility and, in turn, the actual amount of total digestible nutrients derived from the forage. Harvesting forage with relatively high fiber digestibility is economically beneficial whether you feed it or sell it.

Even though yield and stand persistence are always important considerations, keep in mind that at some point in the plant maturity process the forage yield may continue to climb, but the harvested amount of digestible nutrients declines. Many farm consultants are now evaluating nutrient yields per acre in conjunction with total forage yields.

It makes little sense to capture greater yields of fiber that won’t be digested. Though not all livestock classes demand forages with high-fiber digestibility, always striving for such forage still makes good sense. Rare is the year when too much high-quality forage is a problem.

Take some time to sit down and evaluate your 2018 forage quality. If fiber digestibility isn’t where it should be, then try to assess the cause. We can’t change the weather, but factors such as harvest timing and species or variety selection are often controllable.