One of my clients recently changed corn silage bunkers. Both silages tested similar for protein, fiber and starch, although the dry matter content varied a little. He blended the old and new silage for a few days before completely switching over. Even so, you could see that the cows noticed the difference and were adapting. Milk production remained good during the change, but the manure was loose for a few days.
I usually anticipate a change in manure or milk production when switching silage, even if it's the same crop year or cutting. As I discussed using a wine fermentation analogy in my previous column, there are a lot of things going on during the fermentation process. When you open a new bunker, bag or upright, even a subtle change in organic acids and volatile compounds can alter the microbial population in the rumen and you won't know exactly what you have until you feed it.
And although blending silage is recommended, it's also a catch-22 because we are reducing the feed-out rate of both silages. Plus, the packing density at the tapered end of a bunker is lower than the middle. Both of these affect aerobic stability.
Aerobic instability is typically due to the explosive growth of yeasts that occur naturally in all forages, especially corn and cereal grains (and high moisture grains). This can happen right after harvest before all the oxygen in the silage is used up, and at feed-out when the yeast are exposed to air again. The yeasts metabolize the lactic acid in the silage, generating heat. This causes a loss of dry matter and TDN and leads to mold growth. Other silage acids are volatized, changing the smell and palatability of the silage.
Harvesting at the proper moisture, filling rapidly, extensive packing and covering the silage help exclude oxygen quicker. Lactobacillus buchneri is a silage additive that improves aerobic stability by producing both lactic acid and acetic acid. Acetic acid promotes aerobic stability. Adding organic acids like propionic, acetic and benzoic also control aerobic stability.
My client's corn silage was packed very well, but didn't contain either of these additives. Maybe it wouldn't be worth the investment for just a few days while the cows adapted since they didn't lose milk. But I think the manure changes we observed indicate that aerobic stability is always an issue, especially now as hot weather approaches.
Reference: Combs and Hoffman, 2003. Journal Dairy Science 81(Supplement):232.
--Rick Lundquist is an independent nutrition and management consultant based in Duluth, Minn. You can contact him at firstname.lastname@example.org.
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