The whole concept of a feed block is to boost rumen function and efficiency so allowing poorer grade forage to be utilised while allowing ease of management therefore postponing the use of higher quality feed stuffs. If your animals, cattle or sheep, usually need concentrate feeding the chances are that the feed blocks will cut the amount of feed needed and the length of time it's needed but in most cases hard feed will still be required where the animals are under a lot of metabolic stress, peak lactation and just before giving birth. Blocks rarely work out much cheaper than giving a concentrate suppliment that would achieve the same effect but the ease of management makes them much more popular
In this particular instance i'd try and go for a medium grade block or bucket, they tend to be more reasonable on the price. If you are buying just a few as when thry are being needed then as the winter progresses you can buy higher energy blocks to counter the general degredation of the forage available and the increased metabolic tole on the beasts as the third trimester kicks in. If you are selenium deficieint the chances are there will be other minerals lacking as well so I would make sure the blocks are mineralised. It's a real easy way to make sure the cows are in good shape for the early period of lactaion. If you do start feeding concentrates it is still a good idea to leave buckets out, they will still help.
Your cows, being young, are less likely to suffer from grass staggers or milk fever beause they can mobilise their bone mineral reserves really well but with older cows (and sometimes ewes) high magnesium buckets in the early spring, just before the grass begins to really shoot, is always a good idea. Point of caution though with magnesium. High levels of magnesium in feed stuffs can make it taste bitter to the animals so it's a good idea to start the animals of a different feed first until they are into the swing of eating before introducing the high mag.
You aked what type to use. I'd say to phone three or four of your local suppliers and go for the best priced one. Unless you are using a branded product (crystalix or Bridicome etc) The blocks are usually all made in the same factory to the same sort of recipie and then put in different wrappers!
Hope this is of some use to you all.