Not all farm land is suitable for growing vegetables, but it may be suitable for grazing. The big problem we have is that the herds are larger than what the growth on a certain land area can sustain and therefore carbohydrate supplements have to be purchased.
In contrast there's a balanced farming where the area of a farm only have the amount of animals that it can support, no more. Some supplements may be needed even then, but in those cases it's mostly a question of minerals, not carbohydrates.
The amount of water consumed by a bovine is only to some extent wasted, the majority ends up as urine that completes the cycle of returning nutrients to the land where the grazing occurs.
Overall - the major problem with water consumption for beef production is when the farm is unable to support the herd without artificial support.