I'm one of the "complete feed" people. I'm ALSO one of the "free range" people. And I will say again, anything other than a commercially complete feed has the
*potential* to imbalance your chicken's diet.
@NatJ has the right of it, if "free ranging" means turning birds loose in a perfectly manicured residential backyard of zoysia or bermuda grass, or St. Augustine or whatever, surrounded by equaly well manicured lawns and residential pest control, there is a lot of potential for imbalance. Same if their range is acres of sunflower seed, or a corn field, or even a sea of clover. Its magical thinking to believe a balanced meal comes out of a single (or substantially a single) ingredient.
Monocultures are bad, because the chickens have no choice. If they are hungry, they will eat. Monocultures of particularly high energy/fatty feeds (sunflowers, corn, etc) are like sticking sodas, chips, and candy bars in front of teenagers - ruins their dinner.
Feeding something less than a commercially complete feed and then setting your birds loose to free range in hopes it will fix a deficient diet is likewise magical thinking. Unless you have deliberately planted fields to compensate for known feed deficiencies. Which, honestly, is a level of foresight, planning, and execution well beyond the typical poultry owner.
If you are going to free range, and if you plan on their foraging being a substantial portion of their diet, the first thing you have to do is stop pretending. There are *NO* gurantees. There is, however, intelligent risk management. Keeping their complete feed available at all times is a form of risk management - if the nutrition they need isn't currently in their forage (out of season, buried under snow, simply not present), they have complete feed available. Deliberately planting a polyculture - and I don't mean three varieties of grass - but rather a diverse mix of grasses, grains, near grains, legumes, pulses, and all the rest - at least gives the birds a choice - in hopes that they will select during their foraging a mix of plant life (this also attracts a mix of insect life - yum!), and any imbalance will be kept to a minimum, while the period in which the plot is productively "in season" is extended.
Is it nutritiously optimum?
No, probably not. Is it Perfect?
Nothing is. Is it a reasonable response to address the known deficiencies of monocultural plantings? (to say nothing of the way those degrade the soil over time) Yes, I think so.
"Better than the Alternatives" is good enough for me.
and if circumstances or resources make sprouting trays, rather than forage plots, the most practical solution? then I encourage you to follow the same thinking - don't offer the same ingredients, day after day, month after month. Variety helps avoid imbalance.