what are they actually doing behind this miserable word "optimisation"?
Not growing enough wool? feeding the sheep badly? Sheerinig badly?...
Well, no. It's that they haven't found the way to add value, to market properly

While "valorisation" is most commonly used in the sense of "beneficiation", "recycling", "re-use", that is not quite the case here, it seems to me, since that side of the coin refers to getting financial advantage from waste. You are dealing here not with waste, as such, even if from the meat/milk producer's viewpoint it is, but as they say, by-products. Nuance. Of course one man's meat is another man's by-product. The difference is that while a lot of the waste we are now reprocessing and finding economic benefit in or for has always - or for many decades - been just that, waste, the by-products you are referring to have, in the past at least, been valuable items. Less so these days, of course, as we wear synthetic clothes and footwear made with old Coke bottles when we once used leather and wool. Plus ça change, plus ça reste la même chose! On the other hand, looking on the brighter [sic?] side of socio-economic evolution, we now eat prime cuts of beef minced so as to be unrecognizable instead of eating foul bits of sheep offal in a succulent sauce, as we once did, when we got any animal protein at all.

Bref, to insist on the "recycling" nature of "valorisation" here would be to do a disservice to our valiant sheepfarmers by pushing their by-products ever closer to the brink of the waste category, if not directly to the waste dump.

Philosphical considerations aside, what it - as everything - boils down (if your sheep wil excuse the expression) to is money.

How about "poor monetization of by-products"?

Monetization is the process of converting or establishing something into legal tender. It usually refers to the printing of banknotes by central banks, but things such as gold, silver and diamonds can also be monetized. Even intrinsically worthless items can be made into money, as long as they are difficult to make or acquire. Monetization may also refer to exchanging securities for currency, selling a possession, charging for something that used to be free or MAKING MONEY ON A GOODS AND SERVICES THAT WERE PREVIOUSLY UNPROFITABLE [ ... ]
In some industry sectors, MONETIZATION IS A BUZZWORD FOR ADAPTING NON-REVENUE-GENERATING ASSETS INTO THOSE THAT GENERATE REVENUE. Failure to monetize web sites was a problem that caused many businesses to fold during the dot-com burst. Web sites that do generate revenue are often monetized via advertisements or subscription fees.
http://en.wikipedia.org/wiki/Monetization

Mind you, waste gets "monetized" too!
Instead, we earn our keep by helping you reduce, divert and MONETIZE YOUR WASTE FLOW, and by helping you meet your environmental goals. ...
www.oakleafwaste.com/

So do not believe all those that want to export containers filled with your old computers to give to people in the third world. In some cases 75% of the old computers are unusable, they consume more power then there is available and thus the scheme is only using the charity as a means to escape and illegally MONETIZE ELECTRONIC WASTE
http://www.youtube.com/watch?v=5xOcZ_jB1Ok

Launching and sustaining an effective and sustainable city waste management program; MONETIZING WASTE – waste to energy; Promoting public-private ...
www.terrapinn.com/2008/citywaste/conf.stm

Perhaps you could also use severely deteriorating, but I found this a bit strong. or deteriorating market value.