I see this referred to often pertaining to livestock and fish feed but have no idea what it means.
I would guess when you order feed you can specify what % of the feed is protein and what is filler, is this correct?
Different plants have different protein contents, so a protein percentage in a feed depends upon the kinds of plants used in it. For example, buckwheat and millet cut for hay with the grain in would probably contain more protein than the same amount of plain timothy hay. Where the plants were grown, how well they were nourished, when they were harvested, and all that also determines nutrient quantities.
I primarily use soybean meal for a protein source in my feeds. Soybean meal is 44% protein (which is very high for a plant protein). I usually use corn as my other primary ingredient. Corn has protein of 8%.
If I mix 1500 pounds of corn and 500 pounds of SBM, I end up with a mix that is 16% protein overall. I can adjust the ingredients to achieve different protein levels in the finished product. I adjust it for different types of livestock, as their protein needs are different. I also usually add other things to the feed, such as minerals/calcium/salt or whatever else is needed.
Protein sources for commercial feed are whatever is cheapest and legal for that type of livestock. It can be a variety of plant sources, meat and bone meal, fish meal, or the mysterious "animal proteins" which can be just about anything, including ground up chicken feathers or other strange things. They sell the feed based on the level of protein. It is usual practice to vary the ingredients depending on prices of various products, so what you get this time, might change by next time. The protein level will be the same, but what they use to achieve that is not.
If you know the nutrient requirements of the animal you want to feed, you can come up with your ingredients and simply request that they be mixed. If you buy pre-mixed feed, check your labels to see what's in it.
There are not too many fillers in feed. Every ingredient serves a purpose, whether it is to boost protein or just overall energy availability. Almost nothing is totally digestible, so the filler is in the undigestible portions of the feedstuffs. Soybean meal is 90% digestible (by cattle anyways), so there is 10% of just fluff.
Ration formulation is a big topic, I'm no expert and a worse teacher but to take feeding small step beyond simply offering a percent of protein as a ration you really need to understand TDN which as Jena says is the total digestable nutriants. Not just this much protein or that much energy but how much will be available too. Just to add to what Jena said about protein sources, it should be labled on the bag but when fed to a ruminant, protein is just basically nitrogen, so urea (a nitrogen fertilizer) is often used as a protein source as well. There are ways you can boost the protein levels in your hay and grain mixes using feed grade urea, but it can be deadly if done wrong. I'll see if I can find a page I had on feed ration terminology and ration formulation. It's about grade 9 chemistry level science and will explain it better than I can. Balancing a ration by simply adjusting protein levels is a good start and often corrects alot of problems, but taking it that extra step to determine digestability can save you money.
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