You need to have an answer to your “Why” a PHD. While a PHD in agribusiness is definitely possible there are a very few institutions (there is a personal bias here of-course) doing business relevant research in this industry.
This is a good international network to become part of: International Food and Agribusiness Management Association (IFAMA) and understand the institutions and businesses that are interested in the research in this space. While the network does not represent each and every country it is possibly the best network out there
You definitely need to get to review the type of research that is carried out in this field here: International Food and Agribusiness Management Association (IFAMA)