Take a digestibility deep dive

By Mike Rankin

If you’re even a semi-regular reader of eHay Weekly or Hay & Forage Grower, you already know how we feel about the forage world’s reluctance to rid the industry of quality terms derived solely from acid detergent fiber (ADF) and neutral detergent fiber (NDF).

Fitting into this antiquated team of forage quality metrics are the ADF-derived total digestible nutrients (TDN) equation and relative feed value (RFV), which is calculated from ADF and NDF.

We have replacements for both metrics — summative TDN and relative forage quality (RFQ). Although neither is perfect, they do take into account fiber digestibility, which offers a huge leap up.

Let’s just say up front that total fiber content of forage, as measured by NDF, is perhaps the first and most important metric to know. Next on that list has to be fiber digestibility, or NDFD.

As we head into the teeth of fall and reflect on the past season’s forage program, thinking about what went right and what went wrong, it may also be a good time to check those NDFD values, ensure that the TDN values you have are summative, and throw RFV out the barn door in favor of RFQ.

Highly digestible forages can boost dry matter intake and cut feed costs in the form of additional supplements. The advantages can be realized by both dairy and beef producers. 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.

Check your scorecard

Metrics such as NDFD, undigested NDF (uNDF240), total tract NDF digestibility (TTNDFD), RFQ, or a summative TDN calculation are readily offered from many forage labs. If yours doesn’t, perhaps it’s time to investigate using a different forage lab. Also, those old fiber-based metrics are still available at a reduced price. Yes, you get what you pay for.

With the growing season heading into the home stretch, how did your forage stack up for fiber digestibility in 2021? The potential range of results is wide. Remember that grasses generally contain more fiber but also have a higher percent of digestible fiber than legumes, especially if cut early.

Three factors

As you assess the fiber digestibility status of your 2021 forages, keep in mind that three underlying factors will likely explain your results.

Growing environment: This is the one factor that can’t be controlled and in 2021 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.

Although no longer growing, the weather during the forage wilting period can also affect fiber digestibility. It can be severely impacted when cut and wilted forage gets rained on and highly digestible carbohydrates leach from plant tissues.

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.

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. Any grass with the brown midrib trait will offer exceptional fiber digestibility.

Even though yield and stand persistence are always important considerations, as plants mature, the harvested amount of digestible nutrients may actually decline. It makes little sense to capture greater yields of fiber that can’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 2021 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. Also ensure that the metrics you’re getting from your forage testing lab aren’t solely based on NDF and ADF. Fiber digestibility now rules the day!