As our collective digestive systems worked overtime during the past week, perhaps bathroom scales across the nation did the same, indicating a pound or two of fleshy bulk might have been added. Genetic metabolism rates aside, there is some truth to the old saying of “You are what you eat.”
The same can also be said for cows — and perhaps even more so because they perform as they eat. Although animal performance is sometimes limited by genetics, even the best-bred dairy or beef cow won’t function to her potential if she isn’t fed the proper diet.
Most grains and minerals don’t vary significantly from a nutritional perspective, but the same can’t be said for forages, and that — in addition to the essential fiber they provide — is what sets them apart from other diet ingredients. It’s also why there has been a generational decree to quality test harvested and fed forage crops. But even that may not be enough. Just as important is what you test for and how the metric is generated.
Still not there
As an industry, it seems we are still reluctant to rid ourselves of quality terms derived solely from acid detergent fiber (ADF) and neutral detergent fiber (NDF) when forage digestibility (or indigestibility) rules our understanding of ruminant performance.
Fitting into this antiquated list 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.
The replacements for both of these metrics are not new. Both the summative TDN and relative forage quality (RFQ) terms have now been around for years. Although neither is perfect, they do account for fiber digestibility, which offers a huge improvement and better indicator of cow performance.
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. Many dairy nutrition practitioners also like to know undigestible NDF (uNDF240). Nowhere on the list is ADF.
It seems the beef industry gravitates toward TDN, which may be adequate, but again, not all TDN values are created equally and the summative approach is far superior to a solely ADF-derived value. It’s not always clear on the forage test results which value is being reported — you have to ask.
Digestibility drives performance
Highly digestible forages can boost dry matter intake and cut feed costs in the form of additional supplements. The advantages can be realized by all ruminant livestock farmers. 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 is dictated by fiber digestibility and the rate of digestion, and not all forage/hay is created equally in this regard.
As for protein, it is an important component of an animal’s diet, but rarely is it the most limiting factor to animal performance. It is very possible to have high-protein forage that is low in fiber digestibility.
Metrics such as NDFD, 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, even if that lab is associated with your state university. Unfortunately, those old fiber-based metrics are still available at a reduced price, but you get what you pay for.
The emphasis on the growing interest and use of grass forages makes fiber digestibility even more important. Grasses generally contain more fiber but also have a higher percent of digestible fiber than legumes, especially if cut early.
Why digestibility varies
As you assess the fiber digestibility status of your 2024 forages, keep in mind that three underlying factors often explain the final results.
Growing environment: This is the one factor that can’t be controlled. Cool temperatures, especially at night, will have a positive impact on fiber digestibility. This is often why the highest NDFD values are seen for a timely first cutting or in fall-harvested forage.
The weather during the cut-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. Many areas experienced this situation early in this year’s growing season.
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 or grazed before heading, as fiber digestibility declines rapidly once seedheads appear.
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.
Genetics: Species and variety selection is another way to improve fiber digestibility, although to a lesser degree than the previous two factors. 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 improved over previous offerings. Any grass with the brown midrib trait will offer exceptional fiber digestibility.
Though not all livestock classes demand forages with high fiber digestibility, always striving for such forage still makes good sense. Rare is the situation when too much high-quality forage is a problem.
Fiber digestibility continues to rule the day. Ensure that the forage metrics being measured account for this fact. Strive to harvest forages high in digestibility. If you don’t, and your herd doesn’t perform as desired, don’t blame the cows.