Summer covers rather than poor break crop

We've got one or two fields which we planted cover crops in last back end with a view to spring drilling and for one reason or another the covers hardly grew and the wet winter has made the whole scene a bit sad looking. These fields want a break and the idea of slotting linseed, peas or beans in and harvesting slightly more than the amount of seed used, doesn't really appeal.

What we're planning is a summer multi-species cover which we can then mob-graze in Aug/September which will allow our pastures a slightly longer recovery period at a lean time of year and, with any luck, allow us to keep the cattle out that bit longer this coming back end. As well as giving us a cracking, cheap first wheat next year.

So, questions...what to plant and when to drill? What to plant, in my mind, is a mix of whatever is to hand and some C4's like maize, sorghum, millet, sunflowers, buckwheat coupled with stubble turnips, linseed etc. Grasses/cereals? There's a lot of brome/blackgrass plants on ground at the moment. If we wait till they flower and then graze hard or top (or crimp?) and then drill the covers, will that be too late to get the full benefit of the cover (albeit coupled with a feeling of smugness at avoiding a glyphosate spray)?

Any ideas?

Funnily enough it was our agronomist who suggested grazing the blackgrass. We had a very dirty oat/bg companion crop last year which we cut for silage when the bg was flowering and we had almost 100% kill rate. The oats were a different story...we ended up letting them regrow and combining them (bit of a waste of time, but cleared the ground for next crop). Maybe it was the allelopathy of the oats which did for the bg, but done for it was. Similarly, where we grew a summer cover round the Groundswell site, the blackgrass didn't like it. We ended up rolling the cover flat and drilling into it and round-upping after. The following wheat looks so healthy, it's what spurred me on with this idea.

We sprayed off the volunteers and drilled a multi-species cover crop in, on a couple of fields. Got a nice splash of rain and then the slugs came along and ate the whole lot. I have got a bit complacent about slugs, all our other spring crops came away fine, but we were drilling them at 200kg/ha seed rate, the covers were more like 40kg and we didn't roll them in because we got the rain just right. Had a walk out there yesterday, there are a few vetches growing and the odd rye. Will maybe re-drill, want some roots in the ground...