Dithiopyr provides PRE and early POST control of smooth crabgrass, but POST efficacy is often inconsistent on tillered plants. Experiments were conducted to evaluate the interaction of temperature and growth stage on dithiopyr efficacy, absorption, translocation, and metabolism in smooth crabgrass. In greenhouse experiments, I₅₀ (predicted rate to induce 50% injury) measured < 0.14, 0.14, and 0.15 kg ha⁻¹ at low temperatures (average 23 C) for multi-leaf, one-tiller, and multi-tiller smooth crabgrass, respectively, while I₅₀ measured < 0.14, 0.88, and > 2.24 kg ha⁻¹ at high temperatures (average 32 C), respectively. Multi-tiller (three to five tillers) smooth crabgrass absorbed more root applied ¹⁴C-dithiopyr than multi-leaf (three to four leaves) and one-tiller plants, but specific radioactivity (Bq mg⁻¹) was two to three times greater in multi-leaf plants compared to tillered plants. Smooth crabgrass treated at 15/10 C (day/night) had ≈ two times greater specific radioactivity following root applied ¹⁴C-dithiopyr than at 30/25 C. Radioactivity distribution to shoots from root applications measured 43, 30, and 20% of the total absorbed for multi-leaf, one-tiller, and multi-tiller plants, respectively. Smooth crabgrass had two times more foliar absorption of ¹⁴C-dithiopyr at 15/10 than 30/25 C while ¹⁴C losses were greater at 30/25 than 15/10 C. Smooth crabgrass metabolism of ¹⁴C-dithiopyr was ≈ two times greater at 30/25 than 15/10 C, and multi-leaf plants averaged 10 to 20% more metabolism than tillered plants at 7 d after treatment. Results suggest differential absorption, translocation, and metabolism may contribute to dithiopyr efficacy on smooth crabgrass at various growth stages, but use under high temperatures (30/25 C) could increase losses from volatilization, reduce foliar absorption, and increase metabolism compared to cooler temperatures (15/10 C).

Nomenclature: Dithiopyr; smooth crabgrass, Digitaria ischaemum (Schreb.) Schreb. ex Muhl.