Each first handler, in addition to remitting the assessment collected pursuant to § 1280.217, shall pay an assessment equal to forty-two cents ($0.42) per head of lambs purchased by the first handler for slaughter or slaughtered by such first handler pursuant to a custom slaughter arrangement. The rates of assessment for first handlers shall be increased or decreased proportionately if the assessment paid by producers, seedstock producers, and feeders is increased or decreased. Such assessment shall be remitted with the assessments collected pursuant to § 1280.217.