8 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



chance that a calf will be born within 3 or 4 days of the 280th day. 

 With sows it is not quit6 an even chance that birth comes on the 

 113th, 114th, or 115th day. The table below gives the most prob- 

 able day of birth following service on the first day of any month. 

 The date of birth for any other day of service can easily be calculated. 



Probable day of birth. 



Date of birth of young. 



Date of service. 



Jan. 1. 

 Feb. 1. 

 Mar. 1. 

 Apr. 1. 

 May 1. 

 June 1. 

 July 1. 

 Aug. 1 

 Sept. 1 

 Oct. 1. 

 Nov. 1 

 Dec. 1. 



Apr. 25 

 Mav 26 

 June 23 

 July 24 

 Aug. 23 

 Sept. 23 

 Oct. 23 

 Nov. 23 

 Dec. 24 

 Jan. 23 

 Feb. 23 

 Mar. 25 



Ewe. 



May 31 

 July 1 

 July 29 

 Aug. 29 

 Sept. 28 

 Oct. 29 

 Nov. 28 

 Dec. 29 

 Jan. 29 

 Feb. 28 

 Mar. 31 

 Apr. 30 



Cow. 



Oct. 8 



Nov. 8 



Dec. 6 



Jan. 6 



Feb. 5 



Mar. 8 



Apr. 7 



May 8 



June 8 



July 8 



Aug. 8 



Sept. 7 



Mare. 



Dec. 7 



Jan. 7 



Feb. 4 



Mar. 7 



Apr. 6 



May 7 



June 6 



July 7 



Aug. 7 



Sept. 6 



Oct. 7 



Nov. 6 



FERTILITY. 



There are few problems in practical breeding more important than 

 increase in fertility. The losses to livestock farming from failure in 

 fertility have been estimated as enormous. There are two phases 

 of fertility which must be considered — regularity of breeding and 

 number of young at a birth. 



In cattle and horses, in which twins are neither common nor 

 desirable, only the first phase is important. It is generally stated 

 that twins are born in about 1 birth in 80 in cattle. Triplets and 

 even larger numbers are born occasionally. The number of twin 

 births in horses is generally stated as much less than in cattle, but a 

 tabulation of 3,000 births taken at random from three recent volumes 

 of the General Stud Book (English Thoroughbreds) yielded 66 cases. 

 However, in 29 cases the foals were slipped, and only 12 of the 132 

 were born alive. According to this tabulation, twins are born once 

 in 45 births, but only one living horse in more than 200 is born 

 a twin. 



Among the factors which affect fertility in both its phases are age, 

 heredity, climate, feeding, and disease. As already noted, hogs, 

 sheep, and cattle begin to become capable of reproduction when less 

 than half a year old and horses at about a year. Fertility is rela- 

 tively low at first, both in respect to regularity of breeding and the 

 number of young at a birth. There is a rapid rise to a maximum 

 and then a gradual decline. 



The productive period usually comes to an end among light horses 

 of both sexes between 20 and 25 years of age. The corresponding 



