Agave nectar is a sweetener produced from the agave plant. In place of agave here you can use any liquid sweetener (maple syrup, for instance), or if that's not a possibility then a bit of sugar will work as well. I suppose you could leave it out altogether, but it helps to balance the tanginess of the red wine vinegar. I haven't tried the recipe with white asparagus, but I assume it'd work just fine!