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The lucerne sown area has been significantly reduced in recent years due to forage productivity and stand persistence. The production and quality of lucerne can be increased through the adoption of improved varieties and use of P and K. Persistence of alfalfa is largely dependent upon the relative abundance of K (Bailey, 1983). P and K nutrients are associated with persistence and shoot regrowth of lucerne (Simons et al., 1995., Li et al., 1997 and Shelyuto, 1997). The proper balance of P and K is important in maintaining productive stands of lucerne. With a view to these objectives, a field trial was conducted to know the effect of P and K on forage yield, quality, nutrient uptake and persistence of lucerne on a soil, sufficient in P and medium in K with lucerne (Medicago sativa L.). The treatments were taken in factorial randomized block design in combinations of two varieties (GAUL-1 & Anand-8), two levels of P (50 and 75 Kg ha-1) and three levels of K (50, 75 and 300 Kg ha-1). The P and K applied through single superphosphate and muriate of potash, respectively. A uniform dose of N was applied with the treatments. The first cut of lucerne was taken at 60 DAS and subsequent cuts were harvested at an interval of 25-30 days. Totally twelve cuts were harvested in a year. The lucerne green forage (GF), dry matter (DM) and crude protein (CP) yield did not differ significantly due to varieties and P levels in total of twelve cuts. Individual response of P application could not be obtained might be due to sufficient availability of P in the soil. The GF, DM and CP yield increased significantly with the increase of individually application of K from 50 to 150 and 300 Kg K ha-1. The corresponding increase of GF, DM and CP was to the tune of 17.2, 13.3 and 17.6 per cent by the application of 300 Kg K ha-1 over 50 Kg K ha-1. The increase in yield might be due to significantly higher plant height and maximum plant populations in the treatment of 300 Kg K ha-1 as compared to 50 Kg and 150 Kg K ha-1. The interaction effect of Variety x P levels x K levels was significant for DM and CP yield. The significantly higher DMY (220.9 q/ha) recorded by the application of 75 Kg P and 300 Kg K ha-1 to Anand-8 but it was at par with GAUL-1 x 50 Kg P and 300 Kg K ha-1. Application of 75 Kg P and 300 Kg K ha-1 to GAUL-1 produced 17.3 % higher DMY over the recommended dose of fertilizer (RDF) of 50 Kg P and 50 Kg K ha-1. Similarly, application of 75 Kg P and 300 Kg K ha-1 to GAUL-1 produced 17.3 % higher DMY over P50K50. Similarly, application of 75 Kg P and 300 Kg K ha-1 to varieties GAUL-1 and Anand-8 synthesized 28.9 and 15.5 % higher CPY, respectively over P50K50. So these results indicated that RDF of P50K50 is not sufficient for securing the higher forage yield and quality. The application of 75 Kg P and 300 Kg K ha-1 to variety GAUL-1 recorded the highest CPY (45.17 q/ha) and showed significantly superiority to others but it was at par with GAUL-1 x P50 x K300 and Anand-8 x P75 x K300. Application of 50 Kg P and 300 Kg K ha-1 to variety GAUL-1 showed the maximum net incremental cost benefit ratio (1:2.59) followed by P75K300 to Anand-8 (1:2.07).The positive response of GFY was obtained in individual cut (1cut to 12 cuts) of lucerne due to application of 300 Kg K ha-1 over 50 Kg K ha-1 and it was significantly higher in 4th cut, 5th cut, 6th cut, 9th cut, 11th cut and 12th cut by the application of 300 Kg K ha-1. The initiation of regrowth after defoliation was substantially greater when lucerne was supplied with K and the elongation rate of shoots was increased with increased K fertility, which lead to greater mass/shoot (Berg et al.,2003). Individually application of 300 Kg K ha-1 recoded significantly higher K uptake than 50 Kg and 150 Kg K ha-1 and hence recorded significantly higher GF, DM and CP yield of lucerne as compared to 50 Kg and 150 Kg K ha-1. There was a highly positive significant correlation (r =0.95**) between DMY and K uptake. Similarly, CPY and K uptake has also strong positive correlation (r =0.92**). Similarly, the N and P uptake by lucerne were also higher with the application of 300 Kg K ha-1 treatment as compared to application of 50 Kg and 150 Kg K ha-1. Individually application of 300 Kg K ha-1 also showed significantly the highest plant population (296 plants in 12 square m area) after 11th cut as compared to 50 Kg K ha-1 (208 plants) and 150 Kg K ha-1 (247 plants). The lucerne Anand-8 exhibited significantly higher plant population (286 plants in 12 square m area) than GAUL-1 (214 plants in 12 square m area) but it was failed in producing higher forage yield. Individually application of 300 Kg K ha-1 showed significantly higher plant height (65.78 cm av. of 12 cuts) than 50 Kg K ha-1 (61.95 cm) and 150 Kg K ha-1 (63.29 cm). It is concluded that both the varieties of lucerne (GAUL-1 and Anand-8) are more responsive to the higher levels of potassium fertilizers (150 and 300 Kg K ha-1). Moreover, these results specifically indicated that RDF of 50 Kg P and 50 Kg K ha-1 is not sufficient for obtaining the higher forage yield and quality to both the varieties under studied. Combined application of P and K is required to lucerne crop to obtain higher forage yield and quality. Key words: Lucerne, Phosphorus, Potassium, Forage yield and quality, Economics.

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