First, SPCA stands for the Society for the
Prevention of Cruelty to Animals. They certainly are not around to
protect cruelty.
Second, latigo is, once again, showing how horribly misinformed he is when it comes to defamation law.
Proserpina, and tranquility, are right about the standard of proof needed when using a filed complaint as the basis for a defamation action. In addition, there are some states which have no "negligence" standard at all but only an "actual malice" standard for defamation actions, which would then make a showing of actual malice a "requisite element of the tort."
Third, whether something is "per se" defamation or not would be determined by the trial court.
Hopefully the OP will ignore latigo's exercise in unschooled guesswork.
Unless the girl who filed the complaint has made defamatory comments about the farm to others beside the SPCA, the farm probably has no defamation action worth pursuing against the girl.