To all the extension professionals, forage industry folks and farmers who follow along, I'm sorry ! Please don't wince too hard at my inept and extremely over-simplified (but hopefully technically correct) explanations !
1. Grasses and legumes are mostly water when they are very young. For a few weeks in the early spring when the grass is green but not yet actively growing, it's quite possible for a group of animals to be turned out onto lush pasture and not be able to glean enough nutrients to maintain their body weight.
2. As grasses and legumes begin to add height, leaves and stems get added rapidly and nutrient content goes up very quickly. When I refer to nutrients, I'm referring to such things as vitamins and minerals, but also energy content (measured in calories or Megacalories) and protein content. In addition to being relatively copious, at this stage of growth nutrients are also readily available to an animal because the plant hasn't yet began to "harden" off in preparation for maturity.
3. Legumes and grasses reach maturity (defined here as setting seed) at different rates which are species specific (and seasonally dependent). As a general rule, C3 cool season grasses gain physiological maturity more rapidly than do most legumes and most C4 warm season grasses. Available nutrients in C3 grasses also tend to decline more precipitously than either legumes or C4 grasses, but this is offset somewhat because the nutrient content of C3 grasses tends to peak higher than C4 grasses, but still considerably lower than legumes.
4. When we cut forages, we effectively restart the clock and begin all over again. The clock is also influenced fairly heavily by available heat, available moisture, etc. While it may have taken forty or fifty days for grass to achieve physiological maturity in the spring, it may happen in half that much time in the middle of the summer if temperature and moisture are right. Lignification also happens much more quickly in the summer. Thus to achieve the same nutrient profile and availability as first cutting, second and third cuttings ought to be taken at a significantly earlier stage of maturity.
Cutting resets the clock. Providing there is available heat and moisture, grasses and legumes will begin to regrow.
4. Now that we have some understanding of when grasses and legumes offer the most nutrition (while they are still growing) and what we can do to influence nutritive quality (cut it !), we can begin to think a bit about the animals for whom we are cutting and storing this hay.
5. Growing calves/foals and lactating animals of all types require (and can utilize) the highest quality forage. For this type of animal, grass hay should be cut before heads are emerged and alfalfa ought to be cut at mid to late bud, before flowers appear. This might yield an ADF/NDF content of <30 and < 40 respectively, an energy content of 0.9 Mcal/lb or higher, mostly from sugars, and a crude protein content in excess of 20 %. For our purposes, assume ADF and NDF are measures of nutrient availability, and that the numbers I provided equate to very high levels of nutrient availability. That isn't the whole story, but close enough for this discussion.
This field of (mostly) bermuda grass (pic taken this morning) is knee high with no heads visible. If this were any kind of C3 grass, and if this was all I had in the barn, it'd be too rich to feed all winter at our operation. As it is, it's is at the top end of the quality scale in terms of what would be suitable for our retired horses.
6. Horses at maintenance (ie. our retirees) don't require particularly high levels of protein or energy. If I fed the sort of hay I described above, every horse on the place would be fat as a tick and would develop all the metabolic diseases that obesity and old age bring with them. I can't imagine what this stuff would do to an IR or Cushings horse ! As a consequence, I aim a little lower and cut a little later when I'm trying to make hay suitable for what we're doing around here. Especially with C4 grasses which mature and decline relatively slowly, this creates a relatively wide window for getting suitable hay in the barn. I'm aiming for a CP content of 10-12 %, an energy content of 0.65-0.8 Mcal/lb and an ADF/NDF profile of <40 and <60 (moderate availability) respectively.
This field of mixed grass (pic also taken this morning) is at or a little below the lower end of the suitable quality scale for our retired horses. This field will be sold to a farmer with beef cows once it's baled and we'll get another chance at it when it regrows.
Hopefully this will help clarify what we do. There is considerably more knowledge involved in putting up suitable quality hay than just running out to the field and cutting it down at the first available chance.