Grains are composed largely of starch rather than fiber Gleaned from ND Ext.

[Sondra]

Thanks to Lee of Buckrun

Corn can be a problem if they are not used to eating it.  Randomly adding to the diet is what causes most problems with ruminants. They need consistency to thrive.  You must feed small amounts of corn regularly if they are to be able to digest it and get anything out of it.    They can only use what they have developed a microbe population for in the rumen.  Here is some info gleaned from my stockpile on ruminal acidosis.

 "Grains are composed largely of starch rather than fiber. The populations of rumen microbes that most effectively break down fiber and ferment starch are different from one another and prefer different rumen pH levels. As increasing levels of grain are fed rumen pH drops and a resulting decrease in the breakdown and digestion of fiber can occur. (because the ph is selecting for one type of microbe) The negative effects are greatest with low quality forages and higher grain levels which may result in reduced forage intake.The objective is to allow the animal to get maximum utilization and nutrition from forage while using grain to make up a marginal deficiency for energy. By limiting grain the negative effects of starch fermentation on fiber digestion are small. "

I believe this holds true no matter the weather as I lifted this from North Dakota State University Extension site.