For many years, ensiling was considered a great tool to store forages while maintaining nutrient availability. Now we know that keeping silage longer in the silo improves starch availability. Prolonged storage became an important practice for dairy herds feeding corn silage, high-moisture corn, and earlage.
Briefly, the breakdown of zein proteins that surround starch occurs during silage fermentation. This effect is caused primarily by bacterial proteases. Besides, the continuous decline in pH and accumulation of acids as fermentation progresses favors the activity of kernel proteases. A spike in starch digestibility happens during the initial fermentation. This initial spike is followed by a gradual, but less pronounced, further improvement over time. Table 1 exemplifies this effect and has a prediction of the potential implication to energy availability of corn silage.
More time in the silo elevates the soluble crude protein (CP) and ammonia-N concentration of corn silage. This particular response created some concerns of reduced rumen undegraded protein concentration and changes to amino acid profiles in silages. These changes could have potential implications in diet formulation.
No ration concerns
There have not been many studies testing the effect of silage time in the silo on animal performance, but the comparison between fermented versus unfermented corn grain (for example, high-moisture corn versus dry ground corn) could be used as a proxy for these effects. In addition, a study from Brazil reported prolonging storage length of rehydrated sorghum grain silage from 30 to 90 days boosted milk and milk protein yields and reduced milk urea nitrogen concentration. Combined, these results indicate the changes in nitrogen fractions during storage should be of no concern to diet formulation.
We recently completed a study evaluating the amino acid profile in corn silage fermented from 30 to 240 days. Longer fermenting silage durations slightly lowered lysine (0.25% versus 0.22% of dry matter (DM) at 30 and 240 days, respectively), but methionine remained the same (0.13% DM).
Does this reduction in lysine affect diet formulation?
Let’s consider a dairy cow consuming 25 pounds of corn silage dry matter per day to address this question. Based on our study, corn silage fermented for 30 days would provide 28 grams of lysine, but if fermented for 240 days, it would provide 25 grams of lysine.
Mixed results for CSPS
Another benefit of ensiling and prolonged storage is the disruption of the starch-protein matrix during fermentation, which may dissociate starch granules and thereby reduce kernel mean particle size. We conducted four studies to evaluate the effect of ensiling and prolonged storage on corn silage processing score (CSPS; percent of starch passing through the 4.75 millimeter sieve), which was used as an indicator of particle size. A summary of these four studies is in Table 2.
Our initial two studies were in the 2014 corn silage season at the University of Wisconsin. Briefly, fermenting silage for 30 days improved CSPS by 10 percentage units compared with the unfermented corn forage material. Lengthening time in storage from zero to 240 days gradually increased CSPS.
These findings highlighted the potential effects of extended fermentation not only on the chemical but also the physical characteristics of kernels. These results suggest that the known benefits on starch digestibility caused by prolonged silage storage are not solely related to the breakdown of zein proteins but also are related to the reduction in kernel particle size. Based on these initial findings, we suggested a target CSPS at harvest of 65%.
After these initial studies, it was still unclear if the change in CSPS as fermentation progresses was dependent upon the initial values of unfermented samples or some other factors. In addition, it raised some questions about prolonged storage compensating for processing. One of our studies conducted at the University of Florida evaluated the effect of ensiling on CSPS of poorly processed silage. We used samples from 11 corn silage hybrids that were arbitrarily processed to achieve a CSPS of 30% and ensiled for zero or 120 days.
As expected, no effects of ensiling on CSPS were observed. Furthermore, a recent study from Cornell University with four hybrids grown at two locations for two consecutive years did not present a consistent improvement effect on CSPS. These two studies highlighted that prolonged storage length will not replace adequate processing at harvest. Harvesting corn silage with at least 65% to 70% CSPS prior to ensiling is advised.
Fatty acids remained adequate
With the greater interest of establishing high-forage diets, there are more inquiries about changes in fatty acids with ensiling and prolonged fermentation. Overall, the literature data currently available did not detect major changes of long-chain fatty acid profiles.
For example, our laboratory recently presented some data on this topic at the American Dairy Science Association meetings. Briefly, we compared corn silage from three maturities, harvested with two roll-gap settings, and fermented for zero, 30, 120 and 240 days. Silage maturity and roll-gap settings had minor effects on silage fatty acid profile. Storage had very minor effects on oleic and linoleic acids. Ensiling did elevate linolenic acid from 0.12% of DM at harvest to 0.18% of DM after fermentation. Prolonged fermentation also increased the concentration of free fatty acids from 0.98% of DM at 30 to 1.20% of DM at 240 days.
Let us consider the same dairy cow consuming 25 pounds of corn silage dry matter per day to address this effect. The corn silage fermented for 30 days provided 109 grams of free fatty acids, whereas the corn silage fermented for 240 days provided 136 grams of free fatty acids. Overall, research underscores that unsaturated fatty acid profile derived from whole-plant corn forage samples at harvesting would be adequate to formulate diets.
Stored silage is a dynamic system. We know more than we did 10 years ago, but there’s still more to learn. Currently, research does not support negative effects of prolonged fermentation on amino acid and fatty acid profiles. Finally, CSPS may improve over time, but fermentation will not replace proper processing at harvest.
This article appeared in the November 2020 issue of Hay & Forage Grower on pages 10 and 11.
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