I'd be more concerned about mould growth.
This article by the Victorian Department of Environment and Primary Industries is titled "Wet Hay Can = Mould Or Fire". (Note for clarity Victoria state of Australia.)
The article confirms "...excess moisture (rain or plant sap) equals loss of dry matter/bulk and nutritive value AND probably mouldy bales AND possibly spontaneous combustion, i.e. FIRE!"
This article by Agriculture Victoria seems more scientific and less sensational and is titled "What happens to hay when it heats?"
The article confirms "Heating of hay causes losses of dry matter (DM) and nutritive value. The ideal conditions (heat and moisture) in heating hay results in mould growth which further increases losses as well as reducing the hay's palatability due to production of mould spores. Lastly, there is always danger of a haystack fire due to spontaneous combustion."
We'd purchase 75-100 bales at a time to feed horses and by far our greater concern was mould. (That's not to say you shouldn't be concerned about fire.)
Mouldy hay can cause illness in horses.
This article by the University of Minnesota is titled "Do not feed moldy hay to horses".
The article confirms "Horses are particularly sensitive to dust from mold spores and can get a respiratory disease similar to asthma in humans...".
The article also explains that mould can produce mycotoxins. Mycotoxins are a toxic by-product from the natural metabolism of moulds.
The article makes recommendations that include:
- If a mycotoxin problem is suspected, a comprehensive review of animal nutrition and health is essential – i.e. problems blamed on mycotoxins may be other disorders or nutritional issues. Diagnosing a mycotoxin problem is difficult and often involves the elimination of other possible factors.
- The physical dust problem associated with moldy forage can be reduced by feeding in a well ventilated area, mixing with a high moisture feed or wetting the hay, but these will not reduce mycotoxins if present.
Mouldy hay and the presence of mycotoxins may have a negative effect on rabbits, although I'm not certain.
I'd suggest you do a little more research on the effects of mould spores on rabbits, which could include ringing your local veterinary clinic.
This article by Páter Károly and the Department of Nutrition, Szent István University, is titled "Mycotoxins and Other Contaminants in Rabbit Feeds" and appears to have been presented at the 9th World Rabbit Congress in 2008 in Verona Italy.
It seems the effects on the bunny are more severe than the horse...
But firstly it also better explains mycotoxins as "invisible, highly corrosive, secondary metabolites of moulds which may persist in feed and even hay, when the moulds that produced them are no longer present" (Scott, 1990); and "Nearly all of the mycotoxins are cytotoxic, disrupting various cellular structures such as membranes, and interfering with vital cellular processes such as protein, RNA and DNA synthesis" (Guerre et al., 2000).
The symptoms are numerous and often associated with other more common ailments of the digestive and nervous systems - but of all the negative effects on rabbit health, my greatest concern is associated with the statement that "...damage to organs is cumulative over a period of time."
Why? Because it will be difficult to identify that by feeding mouldy hay to rabbits, you may be inadvertently but slowly poisoning your furry little companion and causing him or her severe pain and discomfort and most likely an early demise.
Hence the update... be careful with the condition of your hay and if in doubt, don't feed it out.