By Bill Mahanna, PhD, Dipl. ACAN, Pioneer Global Nutritional Sciences Manager

The recent warm weather has many producers itching to plant corn, and for me, it also signals that the first cut alfalfa and grass harvest will soon be here.

I found myself recounting situations where dairy producers were forced to feed low-quality alfalfa due to weatherinduced harvest and wilting delays and how sugar supplementation seemed to help offset some of the resulting production problems.

This column will review sugar in forage crops and in the cow’s diet, recognizing that many nutritionists will undoubtedly find themselves in challenging feeding situations with at least some of the 2010 forage crop.

The Basics

Simple monosaccharide, six-carbon (hexose) sugars include glucose, fructose, galactose and mannose. Glucose and fructose are commonly found free in nature, whereas galactose and mannose occur in combined forms. Pentoses are five-carbon, simple sugars that are seldom found in the free state in nature.

Plant disaccharides consist of sucrose (one glucose plus one fructose, commonly found in the storage parenchyma of corn stover) and cellobiose (two glucoses, resulting from enzymatic or acidic hydrolysis of cellulose).

Oligosaccharides are simple sugars of 3-10 hexose units in length like cellotriose (three glucoses) and dextrin (five glucoses, produced by the hydrolysis of starch).

Sugar sources like liquid molasses typically contain about 60% sugar on a dry matter basis that consists of about 70% sucrose (Carver, 2004).

Simple sugars form the building blocks of more complex carbohydrates like cellulose, hemicellulose, pentosans and starch. Pentoses typically exist in polymeric forms known as pentosans and are the primary component of hemicellulose. Xylose and arabinose are pentosans present in plant fiber and vegetable gums, respectively.

Cloudy growing conditions reduce photosynthesis, causing lowered sugar content, which can be problematic for silage production due to a slower pH decline resulting from either a lack of adequate substrate (especially in wet silages) or reduced lactic acid production from fermentation of pentoses. Alfalfa harvested in the fall tends to be higher in pentose sugars, further contributing to fermentation challenges.

Values to Monitor

Most nutritionists tend to focus on sugar (and starch) now that it is a relatively common lab analytes. However, non-fiber carbohydrate — or carbohydrate that isn’t neutral detergent fiber (NDF) — is also a commonly reported value defined mathematically (by difference) as 100 - [crude protein + (NDF - NDF crude protein) + fat + ash].

Typical ration non-fiber carbohydrate values range from 35% to 40% of ration dry matter when ingredients are high in sugar and starch and from 40% to 45% when ingredients are low in sugar and starch (Hoover and Miller, 1995).

There is still some confusion as to what are defined as sugars on lab reports and as outputs in ration software. Lab values are typically reported as water-soluble carbohydrate or ethanol-soluble carbohydrate.

Dr. Will Hoover from West Virginia University has proposed that water-soluble carbohydrate is the best estimate of rumen-soluble sugars in forages as opposed to only simple sugars that are soluble in an 80% ethanol solution.

The confusion involves longer-chained fructans that are insoluble in ethanol but quite soluble in water. The debate has raged for years as to whether fructans should be considered a sugar. It is important for nutritionists to discuss sugar methods with their laboratory and understand which approach is assumed in various ration software (Cotanch, 2007).

The difficulty with carbohydrate partitioning methods such as nonstructural carbohydrates and neutral detergent-soluble carbohydrates (NDSC) is that they have been represented as a single calculated (or measured) value that does not address their nutritional diversity.

NDSC constituents vary in their potential to support microbial growth, rates of digestion, microbial fermentation characteristics and the ability to be digested by mammalian enzymes. The lack of practical analytical lab methods to separate NDSC constituents has been a major stumbling block in partitioning these components for use in the field (Hall, 2002).

Researchers are beginning to understand that sugars may not ferment as fast as originally thought and that starch and soluble fiber may ferment faster than previously assumed. This led some laboratories to report fast and slow pools and corresponding rates of digestion as a troubleshooting guide despite the lack of clear chemical definitions of constituents that make up the two pools (Johnston, 2010).

Harvest, Storage Factors

As discussed in a previous column (Mahanna, 2009), cutting alfalfa later in the day as a means to increase sugar content has research that falls on both sides of the debate. Although a.m. versus p.m. forages differ in initial sugar composition, it is not clear whether these differences still exist after drying and/or fermentation due to cellular respiration reducing sugar levels at night or in the portion of narrow-swathed windrows not receiving direct sunlight.

Research in Wisconsin (Undersander, 2003) showed that 11 of 14 Wisconsin farm samples had higher sugars with later-cut alfalfa, yet only one of the 14 had higher sugar levels in stored forage.

Miner Institute research (Thomas, 2001; Thomas, 2007) showed no statistical difference in plant sugars, starches, NDF or in vitro digestibility. While later-harvested alfalfa was numerically higher in sugar and starches, the small differences either decreased or disappeared entirely by the time the forage was at 40% dry matter. The alfalfa mowed in the morning was ready for silage harvest in about nine hours, while the alfalfa mowed in the late afternoon was not harvestable until after lunch the following day.

Many researchers in the Midwest and East believe it makes more sense to harvest early in the day to maximize the hours of drying from solar radiation rather than expose the crop to delayed drying or increased weather risk.

Cornell researchers (Kilcer, 2006) investigated the benefit of wide-swathing to reduce drying time and also reduce sugar losses in the field. Their study showed that narrow-swathing (40% of cut width) resulted in an 18% loss in sugars in the fi rst 24 hours of fi eld curing compared to only a 5% loss in sugars when wide-swathing (90% of cut width).

It appears that the wide swath not only allows for more rapid moisture loss, but the exposure of the cells to upwards of three times more sunlight during the drying process allowed for photosynthetic production of sugars even while the crop was field wilting.

Their studies also show an improved lactic acid: acetic acid ratio in fermented wide-swath alfalfa and grass, presumably because more sugars were available to drive the fermentation process.

Silage management plays an important role in conserving sugars. The dry matter loss in silages is primarily sugar or starch. Reducing silage dry matter shrink losses by five percentage points (e.g., from 20% to 15%) is equivalent to adding about 40 lb. of sugar to every wet ton of alfalfa silage. It makes little sense to worry about sugars on the front end of harvest only to have them lost to spoilage, aerobic bacteria or poor silage management.