Small dog, smaller steps
Transition your Shih Tzu over ten days or more, increasing the new food in smaller steps than you would use for a large dog. The arithmetic of a tiny bowl explains why: in a meal that may be only a handful of food, jumping a stage can double the new ingredient overnight, which is no longer a gradual change at all. Start with a spoonful or two of new food stirred through the familiar meal, and build from there, judging each stage by stools and by whether the bowl is finished willingly. Patience at this size costs nothing and prevents both loose stools and a small dog deciding the new dinner is not worth the effort.
The Shih Tzu factors: appetite, muzzle, and audience
A Shih Tzu was bred as a companion, and many have a companion's relaxed attitude to food — interested, but rarely frantic. Combine a modest appetite with a short muzzle and a small mouth, and a new food can fail for purely mechanical reasons: pieces too large to manage comfortably get left, and the owner reads it as dislike. Check that the new food's size suits a little jaw, and consider softening a dry mix with a little warm water to make it easier to pick up and more fragrant. Also beware the audience effect: a Shih Tzu fed amid fussing, hand-offered morsels, and commentary learns that meals are performances, and performances can always be renegotiated.
Blending a tiny bowl, stage by stage
Mix roughly a fifth new food into the old for the first few days, thoroughly, so the new pieces carry the familiar flavour. Move to a third, then half, then mostly new, then all new, staying extra days at any stage where stools soften or enthusiasm fades. Weighing the bowl's contents, or using the same small scoop every time, keeps the stages honest — at this scale, eyeballing is guessing. Serve meals at set times in the dog's usual spot and lift the bowl after a reasonable interval rather than leaving food down all day, which blurs both the transition record and the dog's sense that mealtime is an event worth attending.
Encouraging a dainty eater without spoiling the plan
Warm the mixed food slightly, feed after a gentle walk, and keep the setting calm and free of other pets hovering — a Shih Tzu who feels crowded may abandon a bowl it would otherwise finish. If a meal is refused, lift it and offer the same blend at the next mealtime rather than producing something different; small dogs are skilled at training households, and a rotation of substitutes guarantees the new food never wins. Most healthy Shih Tzus resume eating within a meal or two once they learn the menu is not negotiable, and the gradual blend means the resumed eating does not punish their stomach for the stand-off.
When a small dog's food strike needs a vet
Small dogs have smaller margins, so lower your threshold for advice: contact your vet if a Shih Tzu refuses all food for a full day, if loose stools persist beyond a couple of days on a stepped-back blend, if vomiting repeats, or if there is blood in the stool. A tiny dog who is off food and also lethargic, trembling, or unsteady should be seen promptly rather than watched at home. Keep notes on which blend stage coincided with any problem, and keep them in KinsLe where Care Teams timestamps each served meal, giving the vet a precise small-dog history instead of a household's best recollection.