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Follicular Ovulation phase

Luteal phase

Follicular phase

Follicular Ovulation phase

Luteal phase

Follicular phase m

Figure 2. Cyclic changes in ovarian steroid and gonadotropic hormones in the ewe.

Animal Breeding

Crossbreeding is one of the quickest and most economical methods for increasing total beef or pork production. Heterosis, also sometimes commonly referred to as hybrid vigor, is the result of crossbreeding. Increased performance and vigor is not limited to animal breeding, but also results in hybrid plant production. Hybrid increases are seen in the first generation (Fl) crossbred animals. F1 animals are the result of crossbreeding two purebred lines of livestock. Increased performance benefits may be as high as 25% for Fl calves. Hybrid chickens and crossbred pigs have contributed to the increased economics of poultry and pork production.

Three general crossbreeding systems are used to produce market animals: (1) Mating females of one breed with males of another to produce a Fl market animal; (2) three-breed terminal crossbreeding, where Fl females produced according to (1) are mated to bulls of a third breed to produce a three-breed market animal with a maximal level of heterosis (alternatively, the Fl female could be mated to one of the original parent breed to produce a backcross market animal); (3) rotational crossbreeding, where the breed of the sire is rotated among the three original breeds. Rotational breedings usually only use three breeds because the heterosis gained by the addition of a fourth breed is small.

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