Tuesday, November 14, 2006 - 1:00 PM

Persistence-related responses of alfalfa genotypes under grazing in tropical Brazil.

Carlos Pedreira, Sao Paulo University, Dept. Zootecnia ESALQ-USP P, Av. Padua Dias 11, Piracicaba, 13418-900, BRAZIL and Maria D.C. Ferragine, Universidad Nacional del Centro de la Provincia de Buenos Aires, Paraje Arroyo Seco, Campus Universitario, Tandil, B7000, Argentina.

Morphological and physiological determinants of persistence of alfalfa genotypes in response to grazing method were studied on a Kandiudalfic Eutrudox in Piracicaba, Brazil.  In a mob-grazing study, under a randomized complete block design, treatments were replicated six times and combined two grazing methods (main plot; rotational stocking simulated by grazings every 4-wk in the "warm season" and 6-wk during the "cool season", or continuous stocking simulated by weekly grazings) and five alfalfa genotypes (subplots). Genotypes (ABT-805, Alfagraze, Crioula, CUF-101 and Pioneer 5432) were agronomically contrasting (dormancy, origin, and grazing tolerance), and were grazed to a 7-cm stubble for 295 days (last evaluations on the 330th day).  Genotypes differed on final stand counts, plant survival, percent alfalfa in pregraze forage mass, and concentration and pool of total non-structural carbohydrates (TNC) and nitrogen (N) in roots and crowns. Crioula and CUF-101 did not survive under either grazing method. Alfagraze had the highest persistence under continuous stocking, although stand counts declined from 246 to 64 plants m-2 (26% survival). Under rotational stocking, ABT-805 had the highest persistence  (250 to 112.3 plants m-2; 45% survival).  Of the two dormant, non grazing-tolerant genotypes, CUF-101 persisted less.  After 210 d of grazing, the mean proportion of CUF-101 alfalfa in the total pregraze forage mass was only 9.6%, a harbinger of stand degradation, which materialized at the end of the experiment.  Mean root TNC pools for the three grazing-tolerant cultivars (Alfagraze, ABT-805, and Pioneer 5432) were reduced by 57% (651 to 281 g m-2) and by 75% (610 to 156 g m-2) for Crioula and CUF-101, by the 214th day of grazing. Alfagraze stored more N (28 g kg-1 OM) in plant crowns than all the other genotypes, together with a higher N pool (55 g m-2), already by the 86th day of grazing