Pig farming today is still carried out in a manner that can be compared to that practiced in Roman times. Marcus Terentius Varro recommended waiting until sows are one year and eight months old before mating them and recommended continuing to breed them until they were seven years old. Today sows can be mated at six or seven months, but it is not recommended to breed them more than three times. Mortality and parasite infections were more frequent in antiquity. Farmers therefore waited until they knew which sows were healthy before breeding from healthy sows. Modern pigs can breed earlier, possibly due to selective breeding but they weaken after the third litter.