Hi and welcome to permies!
You didn't say where you are located.
After reading "Weeds: Guardians of the Soil" I'm thinking that letting whatever weeds want to come up might help restore the land.
http://journeytoforever.org/farm_library/weeds/WeedsToC.html I'm reading other
books now on restoring fertility/the soil and they also mention that weeds can be an invaluable aid - their
roots grow deeper than grass roots, so they can break hardpan and bring up nutrients, they
feed the soil life, and often grow more mass than plain grass, which is added to the soil when they die or drop leaves. The authors claim that the weeds run the necessary repair cycle and when the fertility is restored the healthy grass easily chokes them out so they cannot grow there any more. I've even read that pastures with more kinds of weeds increase the health of the animals that live there - they can self-medicate when they need to.
I'd be worried that the dairy sludge might be setting you back, if it's too rich it might be the same as adding chemical fertilizer, and some dewormers kill off good insects (like ivomectin). I think the first thing you need to do is get more organic matter into the soil to support soil life. If the field has been chemically fertilized, it can be a slow process, it can take 5 years to get the mycorrhizae, earthworms, bacteria, etc. to build up a good population. In the meantime you could mob graze it with electric fencing, just have the cattle pass through once or twice a year, whatever it can support. The seeds in the manure
should create a lot more diversity of species in there.
When it comes time to seed for grass, please don't plant the cheap fescue seeds - I'm battling the endophyte - it can poison unwary cattle and lots of wildlife like quail and doves that eat the seeds. They have endophyte-free strains now you can use instead. If you will rotationally graze the area, you could consider using gamagrass which has very
deep roots and breaks up hardpan nicely. It has air tunnels in the roots to help it grow in even badly compacted clay soil or soggy areas so the roots penetrate much better than regular grass roots. The plants get really big but you're supposed to leave 8 inches or they'll die, so you have to move the cattle off before they eat it down too much.
It might be a good time, while things are relatively open and un-planted, to consider whether you'll want to contour the land with some swales to capture rainwater and direct it to a
pond or hold
water for drought insurance. It would also be a good time to plant
trees for shade and a crop of nuts or fruit. The book Tree Crops (out of print but you can read it
online) says some trees will increase your feed without significantly taking away grass.
http://soilandhealth.org/01aglibrary/010175.tree%20crops.pdf If you have chickens then mulberries might be a good shade tree to put in, for instance. Mine also appreciated black cherries (the wild kind with clusters of small berries). I read that cattle in early spring prefer poplar leaves even to the fast-growing green grass and decided to plant a few rows of poplars and willows in my pasture. They can both be grown from cuttings very easily - you just poke the cuttings in the ground and protect them until they're bigger. In other countries poplars, willows, and mulberries are commonly used as fodder for cattle and they've found they can actually increase
milk production and weight gains during the slow growth season for grass.
The other benefit from planting trees in the pasture is the amount of organic matter they build up from the shed leaves and dropped branches over the years. They are soil builders. And the grass in their shade will grow more into the hot season because it's cooler under the trees so the heat doesn't stop it from growing as much.
Mark Shepard, in Restoration Agriculture says the goal is to have trees spaced
enough that the grass still grows well but to have enough that the cattle don't congregate under one or two of them too much, compacting the soil and over-fertilizing that one area.
