Scoops come in a variety of sizes, and we use the term pretty loosely. It can be applied to anything from a 1/2 cup measuring cup, to a #10 coffee can, or even a shovel! That can make it difficult to communicate to your vet exactly how much you're feeding. Knowing the volume of your scoop can be helpful, however different feeds have different densities. One scoop of sweet feed does not necessarily equal one scoop of pellets.
Not only will knowing the weight allow you to more accurately feed your horse (especially if you aren't the only one doing the feeding!), you will also be sure that he or she is getting the appropriate nutrition. Nutrients (like protein, vitamins and minerals) are balanced in feeds based on the amount that you are supposed to be feeding (ie, their feeding recommendations for the size of horse and work level.) For example, if you have an average 1000 lb horse in light work, and the label on the bag of grain recommends 1/2 lb per 100 lbs of body weight, that means they should receive 5 lbs of grain per day. Let's say, for the sake of argument, that 1 quart of this feed weighs 1 lb. If your scoop is a 1 quart scoop and you're feeding twice a day (2 pounds), then your horse isn't getting all the nutrition he or she needs. In this instance, you may be better off with a different type of grain - one that packs more vitamins and minerals in a smaller volume with less calories (assuming your horse is in good body condition).
So the next time you're asked "How much do you feed?" - be ready! It doesn't take much time, but a few minutes spent measuring out your feed can be very helpful to you, your veterinarian, and your horse!