The first cut of hay (math from ‘moweth’) is in June and the second cut in September (aftermath from ‘after moweth’).
Round bales are the commonest type in wet climates as they shed water better than square bales; but square bales can be can be easier to manage.
The bales can be wrapped in plastic:
i) to keep hay dry when stored outside. Horses are highly sensitive to moldy feed, which can result in both short-term and long term respiratory and gastrointestinal problems, such as colic.
ii) to convert damp grass into silage or haylage. Haylage bales are usually smaller because the greater moisture content makes them heavier.
Silage or haylage are names for the same basic product: ensiled grass, whether in a massive silage pit or a plastic-wrapped bale. That is fermented, high-moisture stored fodder for ruminants (cud-chewing animals such as cattle and sheep, but NOT horses). Pit silage is more risky than haylage because of its generally higher moisture content.
For haylage grass must be cut when it’s young with high sugar levels and preferably later in the day, as the longer the sun is on the grass, the higher its sugar content.
The ensiling process involves a combination of plant respiration and the action of aerobic bacteria which heat the bale, and use up all the oxygen in the bale. The quicker the aerobic phase is over, the better, as valuable nutrients are being used up.
Once the oxygen is gone, the aerobic heating phase ends. Anaerobic bacteria, which do not need oxygen, multiply and ferment plant sugars, as the acidity increases this fermentation stops and the bale is in a stable state, unless the bale is punctured and oxygen penetrates.
If the bale is not acidic enough undesirable bacteria such as Clostridium botulinum and Listeria dominate and spoilage results in sour and foul smelling bales, often like ammonia, with evidence of slime moulds* and it should never be fed out.
All you ever wanted to know about balage: http://www.eyemead.com/x063.htm