There are two fields of thought about pasture seeding. In one field we tear up the existing pasture and start from scratch. This is an especially good idea if the pasture was compacted or completely overrun with unwanted weeds. The other is an additive seeding which keeps the original plants intact and adds just what is lacking.
I have experienced both types. Once upon a time I had just moved to a rental property with my family and our horses. The pasture was chucked full of crap grass that the horses would not eat. I resolved to abandon the pasture with the landlords permission. He even offered to plow up the ground.
Landlord also suggested Roundup for totally killing the existing plants but I had no interest in that. Our plan of attack was to disrupt the original plants and sow some fast growing and slower growing plants.
I knew that the horses would not be allowed onto the newly seeded pasture. Luckily we were able to keep them near without too much hassle. Our plan involved keeping horses off of new pasture seedlings for an entire growing season but it was apparent by mid- summer that the more aggressive plants had taken hold and could use to be mowed (eaten) a bit.
We realized that there could be damaging effects to the non aggressive plants emerging but we were confident that it would balance out itself.
From a nutritional standpoint I would suggest a mix of legumes and grasses. The reason behind this has to do with natural cycling. Pastures go through an annual progression and a much longer cycle of succession.
The constant pressure of grazers eating the most tender shoots means that the plants as a group are not equally nibbled during the season. Some sprout earlier than others and some dry out earlier in the fall becoming undesirable. To avoid chemical fertilizer one can apply heavy legumes when seeding in order to aid nitrogen fixation.
It is important to think of the pasture as an ecosystem. If we were to cuss all of the ‘bugs’ living out there we would be shooting our self in the foot.
Of course there are critters who damage plants but there are also those who break down the horse manure and add air to the soil. With a big picture view of the pasture you will be less likely to apply harsh chemicals in order to stunt or promote any one small part of the whole.