As a consultant and feed manufacturer, I have often been encountered with a basic problem: Piglets refusing to eat an otherwise good feed. This has even happened to my own products, as will I explain below. So, based on my experiences, I have to offer the following for discussion when such a problem arises. I will assume that the original formula is a good one (on paper), designed by a qualified nutritionist, so we begin with a good feed.
This is the most common case, in my experience, and it has been the only time my own products faced the problem of feed refusal. The more oxidized feeds are, the greater the refusal to the point pigs will not even touch the feed. In my case, the feed had been stored for too long under unfavorable conditions without my supervision. Lesson learned.
Unpalatable milk products
I will relate a case where I was called to fix this problem for a large feed manufacturer. The formula was not the best possible, but nor was it the worst I have seen. Feed intake was merely 40 grams per day during the first week post-weaning for 3-week-old piglets. For this age, my goal has always been 250 grams per day. Without touching the formula, I switched the whey supplier and feed intake jumped up to 150 grams per day. That easy.
Too many additives
Again, a case from my work: I was asked to review a piglet feed formula and the first thing I did was remove more than half the additives. The client was skeptical, but I offered to cover his "potential loss" and he agreed to test it. To his surprise, piglets consumed almost double the amount they used to eat before. He is now a close friend.
Wrong soybean products
Soybeans are a great source of proteins (and other nutrients) but, like everything else, they do not come without their own challenges. Picking the right soybean product (and source, as this can also play a huge role) is a challenge on its own. Sometimes soybean meal is good enough, other times a highly refined soy protein product is needed. To this day, I keep explaining not all soybean products are protein.
Burnt or hard pellets
There is a misconception that piglets prefer small and hard pellets, but the opposite is true. To make a strong pellet, without many fines, one has to increase pelleting temperature or use an additive. Sensitive materials like sugar and free lysine will "burn" easily, whereas precooked starches and some milk products will make a super-hard pellet. Both will depress feed intake considerably. Maillard reaction is a favorite topic of mine.