Silage inoculants are the most common additives used in making corn and hay crop silages. The main active ingredients in these products are lactic acid bacteria that help ensure the fermentation in the silo goes in a direction that helps preserve the crop.

While inoculants have been available for many years, there are still questions about whether they are worth using.

Types of inoculants
There are three different types of inoculant products. The most widespread products are the homofermentative inoculants.

These inoculants help guarantee a fast, efficient fermentation that produces mostly lactic acid. Because lactic acid is a strong acid, silage pH is often lower in inoculated silage compared to untreated silage.

A reduction in carbon dioxide production means improved dry matter recovery from the silo. Typical species are Lactobacillus plantarum, Lactobacillus casei, Enterococcus faecium and various Pediococcus species.

Around 10 years ago, Lactobacillus buchneri began to be marketed in the U.S. This is a heterofermentative lactic acid bacteria which produces acetic acid and carbon dioxide in addition to lactic acid.

The strains of Lactobacillus buchneri in inoculants are able to ferment lactic acid to acetic acid. Acetic acid is a better yeast and mold inhibitor than lactic acid. So these inoculants are good for keeping silage from heating in warm weather.

The third class of inoculants is the combination product containing one or more homofermentative species along with Lactobacillus buchneri.

The goal is to have a fast, efficient initial fermentation from the homofermentative strains followed by an increase in acetic acid by Lactobacillus buchneri later during storage.

The hope is to get the best of both worlds: a silage that is well preserved in the silo and stays cool in the feedbunk.

Where is the return on your investment?
There are two areas where an inoculant can provide a return on investment: dry matter recovery from the silo and increased animal production.

Based on a survey of the scientific literature, one can expect a 2 to 3 percentage point improvement in dry matter recovery with homofermentative inoculants.

This survey had one surprise; about half of the animal studies showed improved milk production in dairy cows or rate of gain in growing livestock when fed inoculated silage.

When the inoculant worked, the average improvement in milk production was 3 percent, whereas average daily gain was improved 5 percent. With these numbers, some back-of-the-envelope calculations can help you see the value of inoculants.

If dry matter recovery is improved on average by 2.5 percent, then you will have 25 tons less loss per 1,000 tons ensiled.

A typical cost for a homofermentative inoculant is approximately $1 per as-fed ton or $1,000 to treat 1,000 tons. In this scenario, you would break even just from dry matter recovery if the value of each ton saved were $40.

Today, good-quality silage is generally selling for more than $40 per ton. Furthermore the saved dry matter is all digestible and so worth even more. So one might expect a return on investment from dry matter recovery of up to two-to-one.

On the animal side, let’s assume the inoculant on average increases milk production by 1 pound per cow per day. If milk is $18, the increase in production provides $0.18 extra income per cow per day.

If the cow is eating 60 pounds as-fed silage per day, then inoculant cost is $0.03 per cow per day. So a very modest increase in milk production provides a six-to-one return on the investment in an inoculant.

For all inoculant types, dry matter recovery may pay for the product, but it is improved milk production that can really make an inoculant profitable to use.

So selecting an inoculant that has a good track record for improving milk production or daily gain can make a significant difference in how profitable that inoculant is.

Why would an inoculant increase milk production?
This question has stumped animal nutritionists for years. Homofermentative inoculants do a number of small positive things that should be beneficial to the cow and rumen micro-organisms.

However, the fact that half the published studies show an average 3 percent increase in milk production cannot be explained by changes in any of the normally measured characteristics between inoculated and untreated silages.

We were curious if the differences might occur in the rumen. Some previous work indicated that inoculants sometimes improve silage digestibility. We ensiled two cuttings of alfalfa and tested 14 different inoculants against an untreated control in laboratory silos.

When we measured in vitro ruminal digestibility on dried, ground silages by standard procedures, all of the inoculated silages had similar digestibilities as the untreated silages. Next, we did an in vitro analysis on undried silages, and we got a surprising result.

Ten of the 14 inoculated silages produced less gas than the untreated silages – the opposite of what we were expecting to see. What might that mean?

Figure 1 shows that there are three main groups of products made by rumen micro-organisms: volatile fatty acids, gases and more rumen micro-organisms.

If digestibility of the silage was not affected by the inoculants but gas production was reduced, then acid or micro-organism production must have increased.

We tested four of the inoculants in another trial and made some extra measurements in our in vitro procedure in order to estimate microbial growth.

One inoculated silage was not different from the untreated silage, whereas three inoculated silages produced on average 8 percent more rumen microbes than the untreated silage.

By our calculations, that could support up to 4 pounds more milk per cow per day.

We have since performed a lactation trial on alfalfa silage made with and without one of the positive inoculants, a Lactobacillus plantarum. The diet with the inoculated silage produced about 2 pounds per cow per day more milk.

That diet also had a 10 percent lower milk urea nitrogen concentration, indicating better nitrogen utilization by the cows on that silage. Because rumen micro-organisms are a major source of protein to the cow, the results suggest more rumen microbe production on the inoculated silage.

There is clearly more research needed to understand why an inoculated silage may affect rumen micro-organisms.

However, today we have some confidence that some, but not all, inoculants are affecting rumen fermentation in a way that accounts for increased milk production.

Because milk production is such a key factor in the profitability of using these additives, it should be an important consideration when selecting a silage inoculant. PD

Richard Muck
Research Agricultural Engineer
USDA ARS – U.S. Dairy Forage Research Center