Dry matter yield and chemical content of forage sorghums intercropping with annual legumes

Abstract

Forage sorghum [Sorghum bicolor L. Moench 'Brown midrib' (BMR)] and annual legumes lablab [Lablab purpureus L. 'Rongai' (L)] and mucuna [Mucuna pruriens L. 'Vine 90 d' (M)] are suitable fodder for the dairy industry in Puerto Rico, but BMR harvested at 90 days, usually has low crude protein (CP, 6%) content, which limits its usefulness. There is the possibility of increasing the CP by intercropping legumes with BMR, but this option has been only slightly investigated. This study proposes to compare BMR in monoculture and in intercropping with 'Rongai' (SL) and 'Vine 90 d' (SM), in addition to L and M in monoculture, in terms of total leaf mass (FM) and botanical components (sorghum, legumes and weeds) and chemical (CP and fiber fractions) at two harvest times. The experimental plots (25 m2) were sown in February and August 2008 in a randomized complete block design with five replications. At each harvest time, 2 m2 MF area was sampled 90 days after sowing. Data were analyzed by using the MIXED procedure of SAS and mean separation tests were performed by using F-protected LSD at 95% level of confidence. No significant differences (combining fodder botanical component and weeds) were detected for the total MF between BMR and BMR associated with legumes in monoculture, with mean values of 8.94, 8.81, and 8.42 Mg/ha for BMR-'Rongai', BMR-'Vine 90 d', and BMR, respectively. Overall, there was no significant difference in MF between May and August (8.1 vs. 7.5 Mg/ha). 'Rongai' yield (Mg/ha) did not change between May and August (3.6 vs. 3.0) but was lower in the intercrop (2.74) than in monoculture (3.94). 'Vine 90 d' had the lowest yield of 2.8 and 1.3 Mg/ ha in May and August, respectively. No differences (P > 0.05) in NDF and ADF content between BMR and BMR-'Rongai'-'Vine 90 d', whose values were 60.1 and 63.9% and 40.1 and 46.5%, respectively. However, differences (P < 0.05) were found in CP content between monoculture and intercropping, with values of 14.1, 11.1, and 6.0% for 'Rongai', 'Vine 90 d', and sole BMR, respectively, whereas the BMR-'Rongai' and BMR- 'Vine 90 d' CP were 9.8 and 9.1%, but these last values exceed by three units the percentage in BMR. In conclusion, BMR intercropped with 'Rongai' and 'Vine 90 d' improved the forage chemistry and helps to control weeds while favoring the performance of desirable botanical components.