Hay - Wikipedia

Commonly used plants for hay include mixtures of grasses such as ryegrass (Lolium species), timothy, brome, fescue, Bermuda grass, orchard grass, and other species, depending on region. Hay may also include legumes, such as alfalfa (lucerne) and clovers (red, white and subterranean).[4] Legumes in hay are ideally cut pre-bloom. Other pasture forbs are also sometimes a part of the mix, though these plants are not necessarily desired as certain forbs are toxic to some animals.

In the UK some hay is harvested from traditionally managed hay meadows which have a highly diverse flora and which support a rich eco-system. The hay produced by these meadows is species rich and was traditionally used to feed horses.[5]

Oat, barley, and wheat plant materials are occasionally cut green and made into hay for animal fodder, and more usually used in the form of straw, a harvest byproduct of stems and dead leaves that are baled after the grain has been harvested and threshed. Straw is used mainly for animal bedding. Although straw is also used as fodder, particularly as a source of dietary fiber, it has lower nutritional value than hay.[4]

In agroforestry systems are developed to produce tree hay.

It is the leaf and seed material in the hay that determines its quality, because they contain more of the nutrition value for the animal than the stems do.[6]: 194  Farmers try to harvest hay at the point when the seed heads are not quite ripe and the leaf is at its maximum when the grass is mowed in the field. The cut material is allowed to dry so that the bulk of the moisture is removed but the leafy material is still robust enough to be picked up from the ground by machinery and processed into storage in bales, stacks or pits. Methods of haymaking thus aim to minimize the shattering and falling away of the leaves during handling.[6]: 194 

Hay production is highly sensitive to weather conditions, particularly during the harvest period. In drought conditions, both seed and leaf production are stunted, resulting in hay with a high ratio of dry, coarse stems that possess very low nutritional value. Conversely, excessively wet weather can cause cut hay to spoil in the field before it can be baled. Consequently, the primary challenge and risk for farmers in hay production is managing the weather, especially during the critical few weeks when the plants are at optimal maturity for harvesting. A lucky break in the weather often moves the haymaking tasks (such as mowing, tedding, and baling) to the top priority on the farm's to-do list. This is reflected in the idiom to make hay while the sun shines. Hay that was too wet at cutting may develop rot and mold after being baled, creating the potential for toxins to form in the feed, which could make the animals sick.

After harvest, hay also has to be stored in a manner to prevent it from getting wet. Mold and spoilage reduce nutritional value and may cause illness in animals. A symbiotic fungus in fescue may cause illness in horses and cattle.[7]

The successful harvest of maximum yields of high-quality hay is entirely dependent on the coincident occurrence of optimum crop, field, and weather conditions. When this occurs, there may be a period of intense activity on the hay farm while harvest proceeds until weather conditions become unfavourable.