A farmer has three fields, and keeps a set of horses in one, cows in another and pigs in the last one.
In the field with the horses there is a plentiful amount of grasses covering it's entirety, and is by some margin the largest field; there would have to be significant loses to the grass before the horses would be left wanting. The cows have the next largest field, though half of it's land is covered by hard standing and poor quality soil that yields little grass. The farmer supplements their natural grass growth with hay.
The pigs have the worse lot, they have a large shed and an outdoor area that is devoid of natural food. All the food they get is provided by the farmer in troughs.
One year the weather was particularly bad for the farm, and the horses found that a patch of their large field had not grown grass properly through a combination of early droughts and then late floods on the valley side.
Some of these horses were extremely frustrated, they were used to having any area of the field to graze from, and the complete freedom that afforded. However, when they looked over at the pigs they were incensed.
The pigs, you see, were still getting the same amount of food as before, the amount they needed, from the farmer while the horses had to suffer a reduced (though still surplus) supply of natural grass.
Naturally the horses complained. Worried that nothing would be able to pull the ploughs if the horses got too upset, the farmer scaled the amount of food he provided to the pig's troughs by a similar fraction as the horses had lost in their field.
Wanting to be fair, and avoid any further arguments, the farmer did the same with the hay supplied to the cows. The cows were too pre-occupied agreeing with the horses about the similar loss of the natural grass growth in their smaller field, and arguing about how much more valuable they are to the farmer than the pigs, to notice the implications straight away.
The horses were now happy, they had more food than they would need. The cows were happy for a while, until they realised that they barely had enough food between the natural grass and the hay to go around, and the pigs were miserable...unable to produce healthy young and were poor prospects for the farmer.
But at least everything was done "fairly", so who can complain?