It depends on the other options available to you. ABM graduates are eligible for many of the marketing and operations jobs that normal PGP students are. Choosing an ABM at any of the top IIMs (including IIM Lucknow) is a better option than a general MBA from a tier 2 business school. Even if you take a drop for CAT, it will be a waste of your year. In an MBA, your college tag makes a big difference. In the long run, the IIM tag will benefit your corporate career a lot.
With specifics to IIM Lucknow, Agri-Business Management (ABM) course, you get to study extra subjects each trimester related to Agribusiness. In the final year, there is a restriction in choosing the major subjects.
As for placements, you will be offered the same opportunities as any other PGP student. Agribusiness in general has a very wide scope.