Making use of NDF digestibility analyses for dairy cattle diets

Kirby Krogstad and Barry Bradford<bjbrad@msu.edu>, Michigan State University Extension - March 10, 2022

Once you have a fiber digestibility number from the lab, how should you to put it to use?

for decorative purposes only — Laboratory assays quantifying the digestibility of neutral detergent fiber (NDF) from forages – such as a 30-hour in vitro NDF digestibility assay (ivNDFD) – can be used to help predict the ability of dairy cows to consume additional feed, which ultimately impacts milk production.

The article “Getting the most out of your forage evaluation: Understanding NDF digestibility”, discussed in vitro NDF digestibility (ivNDFD) measurements and how they are interpreted. Here, we consider utilizing this information and whether these approaches deliver a return on investment. After all, if farms and their nutritionists are investing in these assays, we need to make the best possible use of the information we’re paying for.

Estimating fiber digestion in the cow

One reason to measure the ivNDFD of feed ingredients is to predict nutrients absorbed by the cow. Characterizing NDF digestibility theoretically allows nutritional models to predict how much NDF may be digested in the rumen, how much microbial protein is produced as a result of that ruminal digestion, and how much energy may be available to the cow from digestion of that fiber.

Results from ivNDFD analyses may be used within various ration software. Currently, the Cornell Net Carbohydrate and Protein System (CNCPS) nutritional model predicts NDF digestibility of feeds using ivNDFD at 12, 72, and 120 hours of incubation, while forage NDF digestion rates are predicted using 30, 120, and 240 hours. Unfortunately, current evidence demonstrates that including ivNDFD at 30 hours in place of “book values” did not improve predictions of milk production.

Using ivNDFD for forage variety selection and allocation

Why should farmers and nutritionists pay for ivNDFD if it does not improve nutritional model predictions? Because ivNDFD indicates the dry matter intake (DMI) and milk production potential of a forage.

Control of feed intake is complex, but one factor affecting feed intake is gut fill. Forage, a bulky and slowly digested ingredient, is the major contributor to gut fill. However, if forages are digested relatively quickly, the bulky material disappears, alleviating the feeling of being full. This means that the cow is ready to eat again more quickly if she is consuming very digestible forages. Over the course of a day, greater forage digestibility results in greater DMI, and, because intake is a key limitation to milk production, it typically also leads to increased milk yield.

The benefits of feeding corn silage with increased ivNDFD have been well summarized; increasing ivNDFD of forages by one percentage point increases DMI and fat corrected milk production by 0.38 and 0.55 lb/d, respectively. High-producing cows and cows fed greater concentrations of forage benefit the most from increasing ivNDFD of forages, because these cows are most likely to have DMI limited by gut fill. Research on this topic points to impacts that are two or three times greater when high-ivNDFD corn silage is fed to cows with very high milk production potential.

What to analyze

Farms and nutritionists utilizing ivNDFD assays are faced with the question of what time point(s) to analyze. Although some software allows for three to four time points to be entered, that does not mean that paying for analysis of all these time points is justified.

If the proven value of measuring ivNDFD of forages is to predict impact on dry matter intake, we should ask which measure of ivNDFD is the greatest indicator of gut fill. It has been postulated that cows have a maximum 240-hour undigested NDF (uNDF240) intake and that uNDF240 may be the greatest predictor of gut fill potential of forages. There are two reasons to question the biological relevance of this measurement. First, rumen retention time of forage in a dairy cow much closer to 30 hours than 240 hours, and in fact far less than 1% of undigested feed particles would be retained in the rumen for 240 hours.

Secondly, research results indicate that 24-hour or 30-hour ivNDFD may be more predictive of DMI than uNDF240. In a study conducted in Italy, investigators fed four alfalfa-based diets that differed in 24-hour and 240-hour ivNDFD. Dry matter intake was unaffected by dietary uNDF240 but it was affected by 24-hour ivNDFD concentrations. A 3-unit increase in 24-hour ivNDFD resulted in a 10 lb./day increase in DMI and a 3 lb./day increase in milk production.

Valuing forages

Since we know that increasing ivNDFD of forages may increase DMI and milk production, we can give forages different marginal values based on their ivNDFD. In a webinar, Dr. Bill Weiss outlined how measuring ivNDFD at 30 hours can be used to adjust the value of forages. He outlines a 6-step process:

Measure ivNDFD at 30 hours (% of NDF)
Change in ivNDFD = Sample ivNDFD30 Lab average ivNDFD30 of same forage type (alfalfa hay, corn silage, etc.)
Compute the expected change in DMI (lb./d) = Change in ivNDFD × 0.38
Compute the expected change in milk yield (lb./d) = Change in ivNDFD × 0.55
Calculate marginal income over feed cost = Change in milk yield × milk price – change in DMI × ration cost
Convert marginal income over feed cost to change in value/ton of forage = marginal income over feed cost ÷ (forage in typical ration ÷ 2000 lbs.)

A decision support tool to simplify these calculations is now available from MSU Extension. We prepared an example comparing traditional corn silage vs. a brown midrib corn silage hybrid. We assumed that our ration cost is $0.11/lb. of dry matter, milk is $18.00/cwt, and 22 lb./day of corn silage dry matter is fed. We also assumed our traditional corn silage had a 30-hour ivNDFD of 54.5%, while the BMR hybrid had a 64% 30-hour ivNDFD (% of NDF). Using the steps laid out above, we estimated that the BMR hybrid provides $18 more value per ton of silage than conventional corn silage hybrids. The adjustment does not account for differences in grain content, which can also impact performance; however, if the initial value of silage is based on corn grain yield equivalent, this adjustment should help to better balance the value of starch and digestible fiber in a given corn silage.

Click here for article 3 in this series, where we dig into whether it makes sense to incorporate cool-season grasses into alfalfa fields to influence forage NDFD.

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