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We have a new guy in our grazing group, so I gave him the short course on grass varieties and cattle. It's seems worth posting here too.
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The best forage grasses - those that make what is called a "dairy quality pasture" - are the ryegrasses. There are three main types: annual, perennial and Italian.
Annual ryegrass is a wonderful forage with a good balance of protein and energy (complex carbohydrates): very productive but short lived. It can yield from late fall to late spring if well managed with irrigation, then it goes to seed and dies.
Perennial ryegrass is also an excellent forage. It doesn't yield as bountifully as annual rye and it has more protein, so it is a slightly less well balanced food. If you plant the right variety it will yield 12 months of the year, though it grows slowly in the dog days of summer since it is a C3 grass, and also grows slowly in the dead of winter when the days are very short and there is little sunshine.
Italian ryegrass is a biennial. When planted in the spring after the last frost it will yield like annual ryegrass all spring, summer, fall and winter and the second spring, then it will go to seed and die in late spring of the second year. It is the cold weather that triggers its need to breed and die. If it was planted in the fall like annual ryegrass it would live no longer than the annual ryegrass.
Italian ryegrass is the very best forage grass. When planted in a mix with white clover, red clover and birdsfoot trefoil you get a super-duper pasture, but it takes constant management and renovation, and cannot be continuously productive year after year. There will always be a period in summer and fall when some of it has gone to seed and died. You can stagger plantings so that there is always some, but it takes planning.
A mixed sward of perennial and Italian ryegrass is about the best you can do, and there will always be many other grass species in the pasture since there is a seed bank in the soil that would take 20 years or a lot of chemicals to get rid of. Some of them, such as Dallis grass, are just fine since they are C4 grasses that are most productive in the dog days of summer when the ryegrasses are least productive. Dallis grass isn't as nutritious, but it is productive.
Cattle have their seasons too. The type of European breeds that we raise get heat stressed when temperatures go above 75 F. Eating makes it worse since their rumens run at about 104 F due to the furious activity of the bacteria that actually do the digestion. It takes energy to stay cool since they don't sweat, but it makes heat to get energy from forage. They are in a no win situation in summer unless we feed them simple carbohydrates that bypass rumen digestion and go straight to the stomach, and even then it's hard to get good gains when they are heat stressed.
Understanding the seasons of grasses and cattle allows you to plan growth and harvest. For example, it's pretty clear that the best time to fatten and then harvest steers is in spring. Let them gorge themselves on annual ryegrass until late spring or early summer, and then take them to the knacker. The next window of opportunity is late fall, though it isn't as good as late spring.
The reason that late spring is best is that animals must be on a continuous gain of about 2 pounds a day or more for 90 days or more to have good intramuscular fat and be high in omega-3s, DHA, EPA, and CLA - the super nutritious fatty acids. That's easiest to do when the animals are comfortable with the temperatures and the grasses are most abundant and nutritious.
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I should add that this is true for our location but varies with latitude and such. The possibility exists for the development of better forage species in future. The possibility also exists that I am stone ignorant and that there are better species and systems already in existence.