Predicting Alfalfa Quality Using PEAQ

Table of Contents


High quality forage is essential to dairy profitability. The "Predictive Equations for Alfalfa Quality" (PEAQ) is a method used to predict the forage quality of standing alfalfa. This can help us determine that critically important day when we should begin cutting first cut alfalfa hay and haylage.

The benchmark for "high producing dairy cow" quality alfalfa is considered to be 20% Crude Protein (CP), 30% Acid Detergent Fiber (ADF), and 40% Neutral Detergent Fiber (NDF). Many consider NDF to be the primary quality variable to target. NDF and ADF are used to predict intake and digestibility, respectively. Optimum alfalfa NDF for intake and dietary fiber is about 40%. The 40% NDF should be reached at about the mid-point of the harvest. With warm weather, NDF can increase about 0.7 units per day, so quality can drop rapidly. Of course, finding a window of dry weather can complicate things even further.

Neutral Detergent Fiber (NDF)

Research has shown that NDF can vary from one year to the next by up to 10% when cutting is on the same date. The relationship between morphological stage, such as early or late bud stage, and NDF is not always as high as we might think. Cool, wet conditions can delay the onset of bud stage while NDF continues to increase. In a Cornell study, NDF sometimes reached 40% before any buds were visible, while in other cases buds were visible up to 2 weeks before alfalfa reached 40% NDF.

When To Cut?

When to begin cutting is always a difficult decision. Methods used to determine when to cut include the calendar date, stage of development, Growing Degree Days (GDD), Scissors Cut data, and Predictive Equations for Alfalfa Quality (PEAQ).

Predictive Equations for Alfalfa (PEAQ)

The PEAQ method was developed at the University of Wisconsin. It uses the longest stem and the most mature stem to estimate the NDF of the alfalfa in a standing crop. You can use the PEAQ method to estimate NDF by following these steps:

Step 1: Walk through the field and choose a representative 2-square-foot area. Determine the most mature stem in the 2-square-foot sampling area. The most mature stem will be in either vegetative (no visible buds), bud (buds visible on at least one node), or flower stage (at least one open flower on the stem).

Step 2: Measure the length of the longest stem in the 2-square-foot area. Measure it from the soil surface (next to plant crown) to the tip of the stem (not to the tip of the highest leaf blade). Straighten the stem for an accurate measure of its length. The longest stem may not be the most mature stem. Make sure you measure the longest stem, not an average length stem.

Step 3: Based on the most mature stem and length of the longest stem, use the accompanying chart to determine estimated NDF of the standing alfalfa forage. Then repeat these steps in four or five representative areas across the field. Sample more times for fields larger than 30 acres. Average all estimates for a field average.

NDF Estimates Chart

 Table 1. % NDF Estimates in Alfalfa

Length of longest stem from soil to stem tip (in inches) Stage of Most Mature Stem
 Late Vegetative Bud Stage Flower Stage
28.5 29.7 31.4
29.2 30.4 32.0
29.9 31.1 32.7
30.6 31.8 33.4
31.3 32.5 34.1
32.0 33.2 34.8
32.7 33.9 35.5
33.4 34.6 36.2
34.0 35.3 36.9
34.7 35.9 37.6
35.4 36.6 38.3
36.1 37.3 38.9
36.8 38.0 39.6
37.5 38.7 40.3
38.2 39.4 41.7
39.6 40.8 42.4
40.3 41.5 43.1
40.9 42.2 43.8
41.6 42.8 44.5
42.3 43.5 45.2
3.0 44.9 46.5
43.7 44.9 46.5
44.4 45.6 47.2
45.1 46.3 47.9

For example, if the most mature stem is in the bud stage, and the longest stem is 28 inches, the NDF is estimated to be 38.0%.

PEAQ estimates the quality of the standing crop, and does not account for changes in quality due to wilting (respiration), harvesting (leaf loss), and storage (fermentation). These factors may increase the final NDF content by from 2% (haylage) to up to 6% (hay), assuming good wilting and harvesting conditions. This procedure is most accurate for good stands of pure alfalfa with normal growth. The NDF of the grasses in mixed stands will be higher than the alfalfa, so stands with significant grass need to be cut earlier.

Similar charts are also available for ADF and Relative Feed Value (RFV). A PEAQ stick has been developed, that incorporates the NDF estimates onto an easy to read measuring stick, which can be used in the field.

PEAQ is intended to be used as a tool in making cutting decisions, and is not to replace forage analysis and ration balancing. Studies have shown that NDF can be estimated using PEAQ with reasonable accuracy compared to GDD or Scissors Cut. PEAQ is not perfect, but it is likely more accurate than using calendar date or bud-flower stage alone.

For more information:
Toll Free: 1-877-424-1300