Resposta da cana-de-açúcar a níveis de irrigação e adubação de cobertura nos tabuleiros costeiros da Paraíba.

Abstract

This research had the purpose of sludying the reponse of sugarcane crop (Saccharum officinarum L., variety SP-79 1011) to different combinations of water and manuring, at the time of planting, leveis, considering the alcohol gross yield as the dependent variable and the water and manuring factors as the independent variables. The amounts of water and manuring at the time of planting that will promote the higher productions and best economic results were studied. An experiment was installed in the Capim Farm at the Miriri distillery, in the municípalily of Capim, Paraiba State. The treatments studied were the water (W) and manuring (N) factors and the experiment was constituted of a 4 x 4 factorial scheme with completely casualized plots and with three replications. The levles for the total water depth and for the manuring at the time of planting were, respectively, Wo=0, Wi=152, W2=303 and W3=454 mm, and N0=85, Ni=167, N2=305 and N3=45S kg ha'!. The water application was made by a mobile Center Pivot irrigation system. The water amount applied was equal to the evapotranspiration based on the Class A tank subtracting the effective preciphation. The maximum values applied in irrigation intervals of 12 days were equal to the net water depth per treatment: W, (water depth 1=13.8 mm); W2 (water depth 2=27.5 mm, corresponding to the water depth adopted at the Capim Farm); and, W3 (water depth 3=41.3 mm). The highest number of colms per hectare obtained was 92,130 (W=905 mm and 236 kg .ha1 N), which was superior to the lower optimum limit of 90,000 colms ha"1 found in literature. The colms production was significantly influenced by the water and manuring, but not by the interation between these factors. The mean production of colms obtained in the experiment was 77.44 t ha"1, the lowest 48.2 t ha'1 (W=609 mm e N=85 kg ha"')and the highest 103.2 t ha'1 (W-1,043 mm e N=458 kg ha"1). The mean production of the gross sugar was 12.3 t ha'1, the lowest 6,7 t ha"1 (W=609 mm e N=85 kg ha"1) and the highest 16.3 t ha"1 (W=l,043 mm e N=458 kg ha"!). The price of the gross alcohol, water and fertilizers (nitrogen plus potassium) were, respectively, R$ 0.3107 L"1; R$ 1.6355 (mm ha)"1 and R$ 1.1310 kg"1. The fixed costs (implanting and recurring costs) were R$ 360.29 ha"1.year"1 and the variable costs (cutting and transport of colms) were R$ 7.55 t"1. The combinations of manuring at the time of planting (N) in kg ha"1 and of irrigation (W) in mm (Nm=551,Wrn=397) and (N*=307,W*=308) promoted, respectively, the maximum yield of 11,609 L ha"1 and a maximum net income of R$ 1.439 ha'1. The maximum net income was 1.2; 2.6 e 1.1 times higher than the incomes generated by combinations of manuring at the time of planting (N) in kg ha"1 and of irrigation (W), in mm, (Nm=551,Wm=397); (N=376,W=0) and (N=0,W=344), respectively. The combinations of N and W that promoted the maximum net income and the ones that promoted the minimum costs per isoproduct curve (11,000; 10.000 and 9,000 L ha"1), produced incomes 2.0 times higher than the proportionated by the combinations of the leveis No and W0; and, more than 2.8 times superior to the incomes proportionated by the values of the state mean production of alcohol. It was verifíed that the state mean production of gross alcohol yield, estimated in 3,451 L ha"1, represents, respectively, 34 % and 32 % of the alcohol gross yield and a net income by combination (N*=307,W*=308) that promotes the maximum net income.