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I recently was looking at carcass weights in the NAMP buyer's guide and the largest category is "D" 75lbs. and over. It seems today the largest amount of graded lamb is falling into this category.
Here are the last week's USDA slaughter numbers.
CHOICE AND PRIME, YG 1-4 Head 7,332
Weight Head
45-DN 375
45-55# 388
55-65# 723
65-75# 1,366
75-85# 1,902CHOICE AND PRIME, YG 1-4 Head 7,332
Weight Head
45-DN 375
45-55# 388
55-65# 723
65-75# 1,366
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So larger lamb is definitely out there but there is also a fair amount of smaller stuff too. My point here is when ordering lamb be sure to let the purveyor know what size range you want.
Another way to beat the big problem is to create multiple protein plates. Use that monster chop but instead of putting two or three on the plate, put just one and some other lamb items such as a bit of braised pulled lamb shoulder, grilled lamb riblets, lamb sausage, a small osso buco.
New Zealand and Australian products will be much smaller and less expensive per lb. The flavor profile of those is different but if you need to serve that large 4 bone rack roast these may work for you.
The price of lamb is high and the product has gotten larger and it seems to be staying that way for the long haul. That doesn't mean you shouldn't consider putting it on the menu, it just means you may need to rethink your price or amount served.
Photos of carcass courtesy of http://ansc-cpanel.unl.edu/meats/cde2008/lamb%20carcasses/carcass.htm