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Production Papers

Nitrogen Fertility and Planting Date Effects on Lint Yield and Cry1Ac (Bt) Endotoxin Production

^a USDA-ARS, Crop Genetics and Production Research Unit, P.O. Box 345, Stoneville, MS 38776
^b USDA-ARS, Honey Bee Research Unit 2413 E. Hwy 83, Weslaco, TX 78596

^* Corresponding author (bpettigrew{at}ars.usda.gov)

Received for publication December 6, 2005. Early planted cotton (Gossypium hirsutum L.) and varieties expressing the Bacillus thuringiensis (Bt) gene offer improved yield potential. It is not clear whether the current N recommendations remain appropriate for these new production options. The objectives were to determine how varying rates, application timing, and sources of N affected cotton dry matter partitioning, leaf chlorophyll (Chl) concentration, leaf Bt (Cry1Ac) endotoxin concentration, lint yield, and fiber quality. Four N treatments (112 kg N ha^–1 anhydrous; 112 kg N ha^–1 liquid; 112/56 kg N ha^–1 split; 56/56 kg N ha^–1 split) were applied to three cotton varieties (PM 1218BR, STV 4691B, and FM 832) planted on either an early or normal planting date from 2001 through 2004. The N response was consistent across planting dates and varieties for all data collected as shown by the lack of any interactions with these variables. Although N treatments had no effect on lint yield or any dry matter partitioning components, plants receiving the 112/56 kg N ha^–1 split application treatment exhibited 14% greater leaf Cry1Ac concentration and a 3% greater leaf Chl concentration than the other N treatments. Early planted cotton had 5% greater leaf Chl concentration but a 12% lower leaf Cry1Ac concentration than normal planted cotton. Lower Cry1Ac levels may reflect enhanced remobilization of the leaf protein to feed the faster developing boll load of the early planted cotton. Early planted cotton yielded 10% more than normal planted cotton because it produced 9% more bolls. Current N recommendations for normal planted cotton also appear sufficient for early planted cotton.

Abbreviations: Chl, chlorophyll • LAI, leaf area index • NAWB, nodes above white bloom • PPFD, photosynthetic photon flux density • SLW specific leaf weight