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Making use of forage quality indexes

Rebecca Kern for Progressive Forage Published on 30 August 2019

You have just had your hay or forage analyzed, and the relative feed value (RFV) and relative forage quality (RFQ) are lower than expected.

When interpreting the meaning of these lower-than- expected values, you find resources with lots of equations. But what about the actual values which make up these equations? Note this: Producers can make informed production decisions from the values behind those equations.

First, let’s review those equations. Then we can dig into the practical application for forage production decisions.

Breaking down the equations

RFV: The equation for RFV is: (digestible dry matter [DDM] × dry matter intake [DMI]) ÷ 1.29

The constant value 1.29 sets the index so that full-bloom alfalfa has an RFV of 100 for quick reference. Before calculating RFV, DDM and DMI must first be calculated. All values are expressed as a percent of dry matter unless otherwise specified.

DDM = 88.9 - (0.779 × ADF)
Where: ADF = acid detergent fiber

DMI = 120 / NDF
Where: NDF = neutral detergent fiber

So when looking at differences in RFV, the two measurable values are ADF and NDF. These values vary across samples and can be impacted by forage production practices.

RFQ: The equation for RFQ is more complicated. The RFQ index takes into consideration differences in cattle performance on different forages with the same RFV, indicating there are factors aside from the ADF and NDF that affect digestibility and gut fill. The equation for RFQ is:

(total digestible nutrients [TDN] × dry matter intake [DMI]) ÷ 1.29

The equations for TDN and DMI are calculated as follows for RFQ on alfalfa hay.

TDN = (NFC × 0.98) + (CP × 0.93) + (FA × 0.97 × 2.25) + (NDFn × (NDFD/100)) – 7

Where: NFC = non-fiber carbohydrates = 100 – NDFn + CP + EE + ash

CP = crude protein
FA = fatty acids = EE – 1
NDFn = nitrogen-free NDF = NDF – NDFCP

NDFD = 48-hour in vitro NDF digestibility (% of NDF)
EE = ether extract
NDFCP = neutral detergent fiber crude protein

DMI = (120/NDF) + ((NDFD – 45) × 0.374 / 1,350 * 100)

Where: NDF = neutral detergent fiber
NDFD = 48-hour in vitro NDF digestibility (% of NDF), 45 is average.

So when evaluating RFQ, there are more variables to consider. The NFC, CP and FA values increase as RFQ increases. Higher NDFD also results in a higher RFQ value. As digestibility increases, gut fill capacity increases, as does feed intake. Thereby, animal performance also increases. Conversely, NDF has an inverse relationship to RFQ; as NDF increases, RFQ decreases.

The practical side

Accordingly, the major laboratory evaluated values determinate of forage quality are: NDFD, ADF, NDF and NFC (non-fibrous carbohydrates). Understanding the relationship between fibers and simple carbohydrates allows producers to manipulate these values and improve forage production.

First, ADF represents the least digestible portion of forage containing cellulose, lignin and pectin. Understandably, as ADF increases, the feed value is diminished because the percentage of utilizable feed is lower. Second, NDF represents the indigestible, cellulose and lignin, as well as slowly digestible, hemicellulose. The major difference between RFV and RFQ is that RFQ accounts for NDFD, the digestibility of the hemicellulose portion of the forage. This is often reflected by significantly higher RFQ than the RFV for non-legume species.

Finally, NFC represents the sugars and starches in the forage. While it is a calculated value as it pertains to RFQ, it is important because of the inverse relationship it has with the fibrous portions of the feed. Feed with a higher percentage of NFC content has a lower percentage of ADF and NDF content. Hay producers can use this understanding to improve production practices.

Cutting timing

With the goal to improve hay quality, cutting time and height can influence the NFC and fibrous portions of the feed. Throughout the night, plants utilize stored sugars, identified on lab reports as NFC. Therefore, early morning harvest results in lower NFC percentages and higher ADF and NDF percentages. During the day, plants build NFC stores through photosynthesis. Late afternoon and evening harvest results in higher NFC percentages and lower ADF and NDF. So afternoon harvest will result in higher-quality hay than early morning harvest.

Case scenario: For example, the following pie charts show variation in harvest time. The first shows a.m. harvested (Figure 1), and the second shows p.m. harvested (Figure 2).

2016 a.m. harvest - 2nd cutting alfalfa

As I am not a researcher, there are no statistical differences calculated and year effects are not considered, just anecdotal evidence of second-cutting alfalfa harvested off the same field at different times of day.

2017 p.m. harvest - 2nd harvest

These samples came from a customer who had consulted about how to improve hay quality, so cutting height may have been raised as well. The charts show as the producer waited later in the day to harvest, the NDF portion of the alfalfa hay was reduced as the NFC portion increased due to plant accumulation of sugars throughout the day.

Cutting height effects

Producers can also adjust cutting height to impact RFV and RFQ. Lower cutting heights result in a lower leaf-to-stem ratio than higher cutting heights. More fiber is stored in the stem for plant structure, and sugars are concentrated in the leaf where photosynthesis occurs, thus raising the cutting height can increase NFC percentage and decrease ADF and NDF percentages, resulting in higher RFV and RFQ values. Raising cutting height will require some sacrifice in yield, so a balance between quality and yield must be reached.

In conclusion, if you are reviewing a hay report and wondering why the RFV and RFQ are lower than expected, examine the values that go into those index calculations. Are the ADF and NDF values high? Is the NFC value low? How does the NDFD compare to the 45% of NDF average? Understanding these values and how to manipulate them with harvest time and cutting height could improve the quality of hay produced.  end mark

PHOTO: The number of days before a killing frost defines which crops can be planted successfully and may determine how they are harvested; seed availability will also impact decisions for fall plantings. Photos courtesy of Green Cover Seed.

Rebecca Kern
  • Rebecca Kern

  • Animal Scientist
  • Ward Laboratories Inc.
  • Email Rebecca Kern