January KY Dairy Notes

ky dairy news

The University of Kentucky Dairy Extension team circulates a monthly e-newsletter called Kentucky Dairy Notes. The following article “Interpreting a Forage Analysis” comes from the January 2016 issue of Kentucky Dairy Notes.

Interpreting a Forage Analysis

Written by Mickayla Myers, University of Kentucky Dairy Undergraduate Research Student, and Donna M. Amaral-Phillips, University of Kentucky Dairy Extension Professor.

Interpreting a forage analysis can sometimes feel as though you are translating a random code. Despite this, being able to understand a forage analysis is absolutely crucial to understanding the quality of your feed. Understanding quality of forages is important because they are the backbone of the rations for dairy cows and determine how much extra nutrients we need to add into rations.

Here are some of the first things to pick out when you are looking at a forage analysis.

  1. Dry Matter (DM): Dry matter at harvest is especially important for forages harvested as silage, haylage, or baleage, it can determine packing density (ability to exclude oxygen from the chopped or rolled forage) and thus whether a desirable fermentation of the forage will occur. For corn silage, we recommended a range of 32 to 35% DM while alfalfa silage should be at 35 to 40%. Forages harvested as baleage should be 40 to 60% DM. Dry matter will also determine how much of each forage needs to be included in the ration when weighed on an as fed basis.
  2. Acid Detergent Fiber (ADF): ADF is made up of cellulose, silica and lignin. Lignin is indigestible by ruminant animals and as plants mature the amount of lignin increases. Cellulose is found in the cell wall of plants and the rumen bacteria can digest approximately 30 to 40% of the cellulose. Therefore, high levels of ADF in forages mean less digestibility and less energy available for the cow.
  3. Neutral Detergent Fiber (NDF): NDF is the percentage of cell wall material in the forage and is composed of hemicellulose, cellulose and lignin. NDF is very important in forages because it can increase rumination, improve rumen health and increase milk fat. However, remember that some is good but too much can limit intake, energy intake, and ultimately milk production. NDF also can be broken down and used towards energy for milk production. NDF is a more consistent predictor of energy between testing laboratories when compared to NEL (Net Energy for Lactation), which may have been calculated using different equations in different laboratories.
  4. NDF Digestibility (NDFd): Forage NDFd is measured as a percentage of total NDF and describes the digestibility of the forage. NDFd is not used when balancing rations but describes how fast this fiber can be digested by the rumen microbes. The faster NDF is digested, the more feed a cow can eat which can increase her total intake of energy and thus may increase milk production. Generally, as the maturity of a legume or grass plant increases (before harvest), the NDF digestibility will decrease. Environment or growing conditions also plays a role in fiber digestibility. Hot, humid weather has been shown to increase NDFd. NDFd can differ between laboratories, thus results need to be compared from the same laboratory and not between different forage testing labs. The most common way of estimating NDFd is an in vitro procedure. In this procedure, rumen fluid and a buffer are added to the sample and it is allowed to ferment for several hours. There are three different time frames that are used to measure NDFd, which include a 24-hour (h), 30-h and 48-h fermentation. Most nutritionists or producers choose the 24-h or 30-h time frame because it is uncommon for feed to stay in the rumen of a dairy cow for 48 hours. For corn silage, NDFd should be used to compare the quality of corn harvested as silage instead of NDF content.
  5. Crude Protein (CP): CP is calculated by multiplying the nitrogen concentration by 6.25. It is a good indicator of the amount of protein in the forage that can be utilized by the animal for maintenance, lactation, and growth requirements.
  6. Starch: When evaluating corn silage, starch should also be evaluated. Most of the energy in corn silage comes from the starch content and digestibility. A good range for starch percentage is from 24 to 39% for corn silage.
  7. Relative Feed Value (RFV): Another factor to consider when looking at alfalfa is RFV, or relative feed value. Values that are greater than 180 are deemed quality forage.
  8. pH: Another value that can be considered for silages overall is pH. The pH is one predictor of quality of silage fermentation. Normal pH ranges are 3.7 to 4.2 for corn silage and 4.3 to 5.0 for alfalfa haylage.

Average Values for Forages- Does not denote values for quality forage, just average values for tested samples

Forage DM% NDF% ADF% CP% NDFd (30h) Starch% RFV pH
Corn Silage 34 44 26 8 52.3 32 N/A 3.95
Alfalfa Haylage 40 44 34 22 50.1 N/A (<2%) 134 4.65
Legume Hay 90 39 30 21 40.8 N/A 160 N/A
Grass Hay 92 62 39 11 47.6 N/A 89 N/A
Small Grains Hay 89 26 13 12 31 >43 N/A N/A

*N/A is defined as Not Applicable

*Small Grains are defined as oats, wheat, barley, etc.

Values from Dairy One Feed Composition Library from 5/1/2000-4/30/15

*NDFd 30 is a comparison within labs, not between labs.


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