One possible explanation is that the grain is contaminated with some toxic compound or with some disease-causing organism. All five of the animals in this experiment who died had been fed 2.0kg of gain per day, but since illness increased even with smaller amounts of grain, it seems quite possible that smaller amounts increase the chance of death as well. For those animals that live, grain increases the amount of weight they gain. For each extra kilogram of grain fed per day, the final weight of the animal is increased by about 22 kilograms. No differences were seen between males and females in this regard.
Illness also seems to have no effect on weight gain, provided the animal lives.
The fact that feeding grain led to illness indicates that there may have been something wrong with the grain used in the experiment, which means that the results might have been quite different if grain that wasn't contaminated were used. It is difficult to give advice to the farmers on the basis of this experiment. Provided their animals live, they will gain more weight if fed grain, but the loss from some of the animals dying when fed grain may outweigh this benefit. However, if the illness is due to contamination of the grain, it would not be a problem if uncontaminated grain can be obtained.