Sugar analyses of fresh forage can be greatly affected by lab drying method

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Sugar analyses of fresh forage can be greatly affected by how you handle the sample and how the lab dries the sample, says Kurt Cotanch, William H. Miner Agricultural Research Institute forage lab director.

There’s growing interest in both ruminant and equine nutrition to have analyses of both total and individual sugars, such as glucose, sucrose and fructose, he says. For ruminants, the ethanol soluble sugars (ESC) are the standard analyses, where as for equine, the water soluble sugars (WSC) which include the fructans of grasses, are an important nutrient fraction.

“In either case how the sample is dried in the lab can greatly affect recovery of sugars.” Explains Cotanch. Cellular respiration of fresh cut or chopped plant material continues until the forage is either frozen or dried. Even after freezing, loss of sugars can still occur when the sample is then dried. Freeze drying is the gold standard method to preserve sugars and other volatile compounds for analysis of a dry ground sample. However, freeze drying is expensive and time consuming.

For accurate sugar analyses, cellular respiration must be minimized and the sample dried as quickly as possible.

Pelletier et. al., 2010, conducted a trial comparing how lab drying methods affect sugar recovery and other nutrient fractions, (CP, NDF, ADF, NDIN, ADIN, IVTD and dNDF). They compared five sample drying methods:

 1. Oven-drying 55 degrees C, 48 hours (55)

2. High-temp 100 degrees C for 1 hour, then oven-drying 55 degrees C, 48 hours (100C)

3. Freeze-20 degrees C for one month, then oven-drying 55 degrees C, 48 hours (Freeze-thaw)

4. Microwave for 1 minute, then oven-drying 55 degrees C, 48 hours (MO)

5. Freeze -20 degrees C for one month, then freeze-dry (FD)

For the 55 degrees C and 100 degrees C drying methods samples were placed in perforated  plastic bags resulting in forage depth of 6-10 centimeters, which may have slowed rate of drying, possibly allowing prolonged cellular respiration. “In our lab, the standard method of drying bulky fresh forage is loose material at a depth less than 10 cm,” he says.  

Samples of timothy and alfalfa were collected as spring growth (first cut) and summer growth (second cut) over two years, 2007 and 2008. In both years the stage of maturity for timothy was late heading and heading for first and second cuts respectively. Alfalfa was sampled at early flower/bud for first cut and flower and seed pod at second cut.

The freeze-drying method was used as the reference method for all nutrient comparisons. All drying methods resulted in decreased total and/or individual sugars for both timothy and alfalfa across cuttings.

The microwave treatment was most effective at preserving sugars and most similar to FD method, especially for fructose and fructans in the grass, notes Cotanch.

Percent recoveries of total sugars relative to FD method were: 64 percent (55), 83 percent (100), 88 percent (freeze-thaw) and 97 percent (MO). Total N (CP) was unaffected by drying method, however, N fractions were altered by drying. 

NDIN was significantly increased for both timothy and alfalfa across cuttings compared to FD. ADIN showed variable changes due to drying across forage type and cuttings. Freeze-thaw, microwave and 100C methods showed nearly 2-4-X increases of NDIN and 2x increases of ADIN for both forages across cuttings.

Effects of drying method on IVTD and dNDF were minimal, with slight but statistically significant decreases for timothy but non-significant effects on alfalfa. All drying methods significantly increased NDF of timothy across cuttings by nearly 3 percentage units. Alfalfa NDF was not affected compared to FD method.

Cotanch offers these take-home points for handling and drying of fresh forage for analyses:

1. Keep sample cool, do not freeze and dry quickly. Minimize cellular respiration.

2. Microwave pre-treatment followed by standard oven drying is most similar to freeze-drying for analysis of sugars.

3. Microwave pre-treatment, freeze-thaw and 100C increases NDIN significantly.

4. Fresh-chop, pasture, or green chopped sample (whole plant corn included), if sugar analyses are desired, drying method is critical for accurate analyses.

5. Dry hay and fermented forages, where cellular respiration has ceased, drying method may have less effect on sugar analyses, but still worth investigation regarding other volatile compounds, organic acids and effects on N fractions.

Source: July 2011 Miner Farm Report



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