Validating the NAAIC alfalfa grazing tolerance standard test and assessing physiological responses to grazing in a tropical environment

Abstract

AbstractThe North American Alfalfa Improvement Conference (NAAIC) is the curator of a series of standard tests that allow validation of any alfalfa (Medicago sativa L.) cultivar's pest resistance and agronomic attributes. One NAAIC test is for grazing tolerance, an important trait in the tropics. Our objectives were to validate the NAAIC grazing tolerance test and to describe physiological characteristics related to alfalfa grazing persistence in a tropical environment. Treatments were combinations of five cultivars (varying in fall dormancy and grazing tolerance) and two grazing methods, applied from March 2001 to January 2002 (330 d) in Piracicaba, Brazil. Plots were grazed every 7 d (mimicking continuous stocking) or every 28/42 d in spring–summer/autumn‐winter (graze‐rest scheme of rotational stocking), always to 7‐cm stubble. ‘Alfagraze’ (fall dormant, tolerant) and ‘ABT 805’ (fall nondormant, tolerant) had greater survival under both grazing environments compared with the nontolerant checks ‘Pioneer 5432’ (fall dormant, intolerant) and ‘CUF101’ (fall nondormant, intolerant). The widely used Brazilian cultivar ‘Crioula’ showed poor survival, suggesting that adaption did not affect the test, and Crioula is now described as grazing intolerant. Rotational stocking favored persistence of even the tolerant checks. This was related to maintenance of carbohydrate reserve pools, which declined by 80% in roots and by 85% in plant crowns when plots were grazed every 7 d, but only by 46 and 59%, respectively, under the 28/42 d grazing regime. The NAAIC standard test should be useful to screen cultivars for grazing tolerance in tropical or temperate environments. The use of rotational stocking is best for even grazing‐tolerant cultivars.