Understanding Silage Plot Nutritional Parameters

% DM is the resulting feedstuff after 100% of the water has been removed by drying (100% - moisture). Feed analysis for ruminants report nutrients on a DM basis given dairy and beef nutritional requirements are based on DM due to the large variation in moisture among ruminant feedstuffs.

Differences in hybrid entry DM can give an indication if the maturities of plot entries were similar. Increases in starch, in healthy plants, is highly correlated with increases in whole plant DM.

Sugar is found in both the milky portion of the kernel (pre-blacklayer) and in the stover. It is sometimes called water soluble carbohydrates (WSC). Fermentation organisms primarily use sugar (not starch) to produce acids responsible for lowering silage pH.

There will typically be more sugar in less mature plants at harvest. Comparison between hybrid entries to estimate differences in maturity can be evaluated by DM content (lower DM with less mature kernels), lower starch levels and higher sugar levels.

Starch accumulation is determined by genetics and the growing environment the plant receives. It should be the primary nutrition parameter when selecting a silage hybrid being the most energy dense nutrient and contributing upwards of 50% of DM yield and 65% of the energy in corn silage.

Kernels continue to accumulate starch until reaching physiological maturity at blacklayer. Healthy plants should be allowed to mature to at least ¾ milkline to optimize starch yield.

Healthy corn silage plants harvested at the recommended ¾ milkline (to capture more starch) will be higher in DM (e.g. 36-38% DM) compared to plants harvested at only ¼ milkline (30-32%). Taller plants with more biomass will not be as impacted by starch accumulation as shorter plants.

NDF is the total cell wall comprised of the ADF fraction (lignin + cellulose) plus hemicellulose. It is the residue left after boiling sample in neutral detergent solution. If amylase and sodium sulfite are used during the extraction (recommended procedure), the fiber fraction should be called amylase treated NDF (aNDF) to distinguish from original method. If reported on an ash free basis it is termed aNDFom.

A certain quantity of fiber is necessary in the diet, and of the proper chop length (effective fiber) being controlled by ration design and chop length of all the forages found in the diet. Quantity of fiber in corn silage is not as important to nutritionists as digestibility of the fiber (NDFD) or the level of undigestible fiber (uNDF) that contributes to lowered intakes.

NDF levels will be diluted (reduced) in samples containing more sugar/starch and should not be a hybrid selection criterion.

A measurement of the NDF (neutral detergent fiber, or total cell wall) digestibility typically measured by in vitro (test tube) incubations with rumen fluid at varying incubation times and reported as % NDFD (as a % of total NDF). The most popular single timepoints used by nutritionists to compare samples is either 24 or 30-hour NDFD. Multiple time points are often generated to create a digestion curve from which digestion rates (Kd) can be calculated.

While of great interest to nutritionist when balancing diets, NDFD should not be a primary hybrid selection criterion as it is influenced three-times more by growing environment than genetics. There is minimal NDFD differences between hybrids grown in the same environment, chopped at the same height and harvested at a similar maturity stage. The small 2-3 point difference in NDFD among hybrids is within the error of the lab method and not typically biologically significant to the cow by the time the corn silage is included in the TMR with other feedstuffs.

While not a primary hybrid selection criterion, it is very important for nutritionists to know the NDFD when balancing diets to account for the effects of the growing environment (primarily moisture, nitrogen fertility and late season diseases) experienced by the hybrid. Unlike starch digestibility, fiber digestibility remains essentially unchanged over time in fermented storage.

uNDF240 is the neutral detergent fiber (cell wall or lignin + cellulose + hemicellulose) that is not digested after a certain number of hours incubated with rumen bacteria. uNDF is reported as a % of DM (not as a % of the NDF) with typical rumen retention times of either 24, 30, 120 or 240 hours.

Nutritionists use uNDFom30 or uNDFom240 to estimate when the level of undigested fiber gets so high in the total diet that animals begin to decline in dry matter intake.

uNDF can be thought of as the opposite of NDFD, and like NDFD, should not be a primary hybrid selection criterion given that it is also controlled three-times more by growing environment and harvest maturity than by hybrid genetics.

Calculated by multiplying the total nitrogen in the feed by 6.25, based on the assumption that 100% protein contains 6.25% nitrogen.

Protein should not be a silage hybrid selection criterion because hybrids do not differ significantly in protein content. Nitrogen fertility is a key driver of silage protein content, and the amino acid composition of corn protein is of poor quality (low in lysine and methionine). This is why nutritionists utilize soybean or canola as sources of these limiting amino acids.

A corn silage index that estimates the pounds of milk (or converted to beef gain) produced per DM ton of forage based on University of Wisconsin (MILK2006 or MILK2024) calculations.

There are several assumptions built into these kinds of indexes regarding fiber and starch digestibility which may not appropriately rank hybrid genetic potential before introduced to the influence of varying growing environments and harvest timing.

This is an in vitro (test tube) rumen fluid (or enzymatic) starch digestibility analysis. Sample grind size (1-4mm) and incubation time (2-10 hours) vary by laboratory, but commonly presented as 7-hour starch digestibility.

This is not reported on Pioneer reports, nor in any University silage trial reports because research has shown minimal differences exist between similar maturity hybrids grown in the same environment.  It also only represents ruminal digestion and does not account for intestinal digestion.   

It is also well documented that ruminal starch digestion increases over time in fermented storage due to microbial action solubilizing the protein (zein) which surrounds kernel starch granules.

Starch digestibility is an important parameter for nutritionist to balance diets after a hybrid is exposed to growing environment and harvest management, but similar to NDFD, it should not be a selection criterion to rank hybrid genetic potential.

When selecting or ranking a silage hybrids’ genetic potential, prior to the influences of non-genetic factors such as growing environment or harvest management, it is best to consider agronomic strength/weaknesses, DM yield and starch content before considering other nutritional traits or indexes.