Piglets will always go for a sweet feed. That is, if they are given a choice, or if the alternative contains a foul-tasting or bitter ingredient. Piglets also love a sour taste, especially that coming from acids giving a taste like lemon or vinegar. Both facts are well known to nutritionists for many years, and why deny it -- many of us like sweet and sour pork from Chinese cuisine!
I have recently had a discussion with an Asian colleague about the fact that there is no literature on the effects of combining sweet and sour ingredients and testing such feed in weaned piglets. I must admit I have been using this combination in my own products and in many client formulas for some time now, but real, hard evidence is lacking, indeed.
I will only add a few more words on feed preference tests. From an academic point of view, it is interesting to offer animals (pigs, in this case) two feeds simultaneously and let them choose the one they like best. But, in practice, we don’t do this. In fact, trials conducted in the 1970s demonstrated that pigs, when not given a choice, will consume both feeds (from a previous preference test, in which they picked their preferred flavor) at similar levels and grow without difference. So, if anyone is going to test this sweet-and-sour idea, I would argue against doing an expensive choice test. Instead, a simple 2x2 factorial trial would suffice for the lot of us who formulate piglet diets on a daily basis. And, if the results are positive, let me know.