why does my weed smell like hay? my first grow and...

Dried too fast and or put into jars too quickly. Ideally you should hang entire plant and be at 60F at 60% humidity. Goal is a long, slow dry, so that when it goes into jars it's basically done. No need to burp if dried properly.

The Relative Humidity of the drying room was too low. Drying takes between 10 days on drying racks to 20 days if branches are hanged. The process must be done at low temperature, around 18° Celsius and 62% Relative Humidity, away from direct light.

the easiest and simplest way to never f up again curing is buying grove bags. hang the plant, after like a week or 2 trim the weed, put in the bags and the bag will do everything for you. no burping, no boveda packs etc. Thats actually the best and easiest way i've ever cured. (Some also dry and cure in those bags but i dunno, never testet that myself) ;;

in addition to what other shave said, you can also dry for to long or to hot. Terpenes are volatile and give weed its characteristic aromas. The plant will not produce any more terpenes after its harvested and if the temps are high the terpenes with low boiling points will off gas much faster. UV/Light & High RH can also affect these outcomes. High RH because it encourages moulds and they will rot out the cannabis and UV because it will degrade the Terpenes. Time is the last factor, even if your environment is perfect in terms of temps. rh and no light, Drying to long will cause more and more of the terpenes to off gas and loose the smell your looking for. I dry for about 7-10 days and then trim it all up and put it into jars to cure, I store the jars out of light and seal em up, burp them as needed "more at the beginning, less as it ages" and its smells will mature. If it starts to smell like cat piss of ammonia it means the buds where not dry enough and need more time outside the jar. The microbes will help cure or ferment in a way the buds in the jar and provide a smooth smoke and pleasant experience. I cure my buds for a min of 3 months before consuming unless it going to be made into Hash or edibles then w/e works. One option to help fix this is a humidity pack infused with terpenes. Some companies have released humidity packs that have terpenes infused into the pack and when you store your weed and cure it with this it gains some extra smells. Its kinda a trick for not doing a good job but its an option if you want a quick fix. I myself would not use this and just turn it all into edibles. infuse some coconut oil or butter and cook with it. Good Luck!

If the drying space has no ventilation and lacks fresh air circulation you're very likely to develop that hay smell. If the buds are close to each other humidity may stagnate and will cause the hay smell. Over 60% RH humidity will easily cause the hay smell, especially in a warm environment.