THE BREEDING SEASON 59 



58. The oestrum and lactation. In many of the 

 domestic mammals, the period of heat is influenced by 

 lactation. Nursing the young may prevent entirely or 

 retard the appearance of heat after parturition. Ewes 

 seldom come in heat while suckling young. In the case 

 of cows, the oestrum is retarded. There is much varia- 

 tion in the length of the time elapsing between the birth 

 of the young and the first appearance of heat in the cow. 

 It is probably about sixty days, although it may recur 

 earlier or later than this period. The sow will generally 

 come in heat three days after giving birth to a litter of 

 pigs, but the oestrus period does not again occur until 

 after the pigs are weaned. A prominent breeder of Duroc 

 Jersey hogs, S. Y. Thornton, says, " In many cases, a 

 sow that is in good condition will come in heat the third 

 day after farrowing. I have bred them at that time but 

 seldom knew one to get with pig if she was suckling, but 

 one that has lost her pigs will invariably get with pig 

 from the first period which is usually the third day after 

 farrowing. A sow will often come in heat when her 

 pigs are four to six weeks old if she has been well fed." 

 The distinguished breeder of Berkshires, A. J. Lovejoy, 

 says, " Sows often show signs of heat on the third day 

 after farrowing, and again at eight weeks after farrowing, 

 while suckling. We find that after weaning a litter, 

 a sow will usually ' come in ' in three to five days.'" 



The heat period in the mare is very irregular. The 

 " foal heat " occurs in seven to nine days after foaling. 

 The oestrum recurs in most mares throughout the nursing 

 period. But some mares do not come in heat during the 

 time they are suckling foals. It is a well-known fact 

 that there is a strong tendency in some mares to breed 

 only once in two years. In some of the smaller mammals, 



