I was reading fairly recently ( The times week end editions perhaps ) that these spears can be viable ones.
Evidently it all depended of when you set the crown in the ground and how /when the first harvest were taken off .
It is being practiced by commercial growers here in the UK with tunnel grown stuff .
I can't remember if if knackers the plant though or if they beft a percentage of the spears to grow on ten cut the ferns as normal & give a final high potash compost covering a few weeks after cutting the ferns .
You may find some of the big seed houses are sell two crops a year asparagus crowns ..could be worth a look at a few for more info .