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Late-rotation fertilization was studied over a 6 yr period in loblolly pine (Pinus taeda L.) plantations in north Louisiana. Fertilizer (150 lb/ac of nitrogen and 50 lb/ac of phosphorus) and herbicide (hexazinone, 2.5 lb ai/ac) were applied to five loblolly pine plantations in the 25-yr-old age class (25-26 yr), and five plantations in the 30-yr-old age class (30-32 yr). The study was established as a randomized block design with three blocks in each plantation. Three treatments studied were fertilizer (F), fertilizer+herbicide (FH), and herbicide (H), which were compared to a control. Diameter and volume growth responded significantly to treatments F and FH in both age classes. A greater per-tree response occurred in the 25-yr-old age class as a result of the additive effect of recent thinnings. The use of herbicide in treatments FH and H did not cause a significant increase in growth response. The growth response in treatment FH was not significantly different from the response in treatment F, nor was the growth response in treatment H significantly different from the control. In the 25-yr-old age class the response to fertilization (treatments F and FH) was allocated primarily to the larger trees. Treatments F and FH increased total volume growth (lb) by 48 and 55 ft3/ac/yr, respectively, over control plots while treatment H increased growth by 15 ft3/ac/yr. In the 30-yr-old age class, the response was distributed more evenly among all diameter classes. Treatments F and FH increased total volume growth on average of 58 and 60 ft³/ac/yr, respectively, over control plots while treatment H increased growth by 17 ft3/ac/yr. An economic analysis was performed using discount rates of 8% and 12%, and current prices and costs at the time of the study. Rates of return ranged from 29.6 to 38.8% under various economic scenarios (discount rates, prices and costs) in the 25-yr-old age class. The net present value equivalent to an annual income (EAI) gain for each of the 6 yr of the study ranged from $34.07-$60.54/ac/yr. These same economic scenarios produced rates of return of 23.3 to 31.9% in the 30-yr-old age class, with an EAI range of $18.98-$40.03/ac/yr. South. J. Appl. For. 24(3):166-175.

Response of Loblolly Pine Plantations to Late-Rotation Fertilization and Herbicide Applications in North Louisiana