I currently am a contractor in the gas industry I would like to start selling hay on the side can I do this and write the expenses off and file as a farm?

[Critter]

There is nothing that stops you from selling hay and reporting it on the Sch F ... make sure to keep accurate records of all income and expenses for this venture. 

[Anonymous]

one thing to consider is will this side business produce a profit  3 years out of 5.  if not, the IRS might deem this a hobby and in that case disallow expenses in excess of revenue.  say you don't habe the 3 out of 5 and the IRS deems it a hobby.  you can provide evidence that it's not.  such as a business plan,  hours worked at this,  early year losses due to start up and other factors, value of farmland increasing more than losses.   

here some further guidance

https://www.law.cornell.edu/uscode/text/26/183

https://www.taxdebthelp.com/business/irc-183-hobby-loss-rule