For one, they are short-term, denying that nutrition can have an effect beyond the few weeks used in a feeding trial. Undetected nutrient imbalance in youth has been shown to affect susceptibility to many chronic degenerative diseases which appear later in life (far beyond the scope of the few weeks of a feeding trial). Even the health of future generations can be affected by diet through transference of nutritionally-induced genetic weakness and epigenetics.
In the past, animals being fed "complete and balanced" pet foods, as determined by the food meeting certain nutrient percentage requirements (National Research Council [NRC] requirements), have succumbed to disease. So feeding trials were introduced. But foods that have passed feeding trials have produced disease and death as well.
An AAFCO feeding trial requires a manufacturer to send food to a laboratory where it is fed to caged laboratory breeds for a period of 26 weeks. Hair coat, weight, body measurement, and a few blood tests are run to determine nutritional perfection - "100% completeness." Only six of the eight animals in the test need to pass in order to stamp the food complete and balanced.
Such a testing method is like trying to measure the length of a virus with a yardstick. Long term robust health and genetic fitness simply cannot be measured by such a short term simplistic test.
"Caged" human prisoners of war have survived for years on little more than water and rice. Survival, or passing crude measures of nutrient deficiency, does not equal nutritional "completeness" (perfection).
Additionally, results from an unfortunate laboratory-bred puppy or kitten living on concrete or in a stainless steel cage, under fluorescent lights, breathing conditioned air, do not correlate to real animals in homes and back yards.
Anyone concerned about the welfare of dogs and cats must also object to the unnecessary cruelty of imprisoning animals for months and years, testing them with every manner of food to see if they will get sick. Particularly is this so since feeding trials do nothing more than perpetuate the complete and balanced myth.
Feeding trials simply give a false air of science to the "complete" claim. Having the AAFCO feeding trial stamp of approval on a processed pet food you are feeding meal after meal, will do nothing for the health of your pet. You, taking charge of feeding using the common sense you use with your own meals and the wisdom of nature, is the only path to the best pet food.
Thought for the day: "Dogs' lives are too short. Their only fault, really." – Agnes Sligh Turnbull
Word for the day: longevity - noun: length of life span. The trick is to make health span equal life span (longevity).