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Montana Farm Flock Sheep Production Handbook
Repro
duction - Section 4 of 7
By Rodney Kott, Extension Sheep Specialist, Animal & Range Sciences Department, Montana State University

Reproductive rate or the number of lambs marketed per ewe in the breeding flock is the major factor influencing profitability. The average reproductive performance of a typical whiteface range ewe is shown in figure 8. Notice the stages where the majority of the reproductive wastage occurs. In order to optimize reproductive efficiency a sheep producer must understand the basic stages of the reproductive cycle of a sheep.

 

Average Reproductive Performance of a Typical 

Whiteface Ewe Under Range Conditions

 

Ram Fertility Ovulation Rate 1.7 Embryo Survival

(Embryo survival at ovulation rates of 1.7 or less accounts for less than 5% of the genetic variation in litter size).
Lamb Mortality Lambing Rate 1.5

(In most cases where increased lambing rate is the selection objective, ovulation rate will be the primary source of genetic improvements).
Weaning Rate 1.3

 

Puberty in the ewe lamb is the point in which she first exhibits estrus. In typical western range sheep operations ewes usually do not exhibit estrus the first year and thus first lambing occurs at two years of age. Farm flocks are increasingly shifting to lambing first at one year. Breeding ewe lambs will increase lifetime productivity of ewes, but it requires a higher level of management and feeding than if ewes are handled more traditionally. Breeding a ewe as a lamb can often increase her lifetime lamb production by as much as 15 to 20 percent.

However, breeding ewe lambs is not always economically advantageous. If ewes are to be successfully bred to lamb at 12 to 14 months of age, nutrition is critical. Sheep operations that have a high dollar input per ewe (intensive) are more likely to benefit from breeding ewe lambs than range sheep operations (extensive) where dollar input per ewe is normally lower.

Ewe lambs can be mated successfully without detrimental affects on subsequent reproductive performance providing they achieve a threshold body weight within the breeding season. However, ewe lambs that are to be bred to lamb at one year of age will require special treatment if success is to be achieved. Many management factors affect successful breeding of ewe lambs. Because numerous factors influence conception rates among ewe lambs, it is possible for some sheep producers to get as high as 95 to 100 percent of their ewe lambs bred while others only get 10 to 40 percent bred.

Age at puberty is influenced by both breed and nutrition, as they influence growth rate of the lamb. Good nutritional management is necessary for lambs to mature and develop sexually. It is important to develop a realistic and sound feeding program to insure success and high fertility. There is no single correct management program for breeding ewe lambs. Management programs will vary depending on the goals and objectives of the manager and farm or ranch resources (labor and feed availability).

In general, ewe lambs must weigh approximately 65 percent of their mature body weight at the start of the breeding season in order to insure a high percentage of them breeding. However, for our more traditional breeds such as Rambouillet, Targhee and Columbia, a target weight of 70 percent of their mature body weight will produce more satisfactory results. In contrast, breed and/or breed combinations that contain one-quarter or more Finn breeding (Polypay) can probably get by with a target weight of 60 to 65 percent of their mature body weight. For example, if a sheep producer raises Columbia sheep and ewes have a mature body weight of 165 pounds, ewe lambs should attain an average weight of 115 pounds at the start of the breeding season (165 lbs. X 70%). With good management, this should produce conception rates of 75 to 90 percent. If ewe lambs are one-quarter Finn and mature ewes weigh 145 pounds, they would need to weigh between 87 and 94 pounds at the start of breeding.

Range sheep producers often feel that it is not economically feasible to feed ewe lambs well enough to reach the desired target weight at the start of the breeding season and may accept a lower target weight. Producers making this management decision must be prepared to accept lower conception rates. Many times in range operations the biggest lambs are singles and therefore selecting those ewe lambs that breed as lambs may encourage selecting for singles rather than multiple-birth ewe lambs. This would have a detrimental impact on overall flock prolificacy in the future.

Reproductive Cycle of the Ewe

A ewe's reproductive timeline is depicted graphically below. The critical time periods include the last month of pregnancy (nutritional stress) and the first month after lambing (period of environmental adaption for the lamb). Again, periods in which reproductive wastage occurs are between ovulation and implantation for embryonic loss, between implantation and parturition for fetal loss and between parturition and weaning for lamb loss.

Reproductive time line for sheep

Breeding Season: Sheep are seasonal breeders or in other words will only breed during certain times of the year. Although the initiation and end of this period varies with breed of sheep and temperature, it tends to be most affected by day length. Basically sheep breed in decreasing or short day lengths. Common domestic sheep breeds have breeding seasons of five to seven months. In Montana, generally a few ewes will start cycling in September. By October all ewes will generally be cycling (see figure below). If some ram lambs have been left intact for replacements as is the situation with most purebred sheep operations, it is recommended that they be weaned by mid to the end of August to avoid them breeding some ewes. Peek ovulation rates generally occur in November.

Effect of Time of Breeding on Reproduction

Estrous Cycle: In sheep the length of estrous or time between periods of standing heat is about 17 days. Estrus or standing heat lasts about 30 to 40 hours with ovulation occurring during the last half of heat. A ewe, once she starts cycling in the fall, will cycle or come into heat about every 17 days. During a 35 day breeding period a ewe should have had the opportunity to cycle and be bred two times. A 60 day breeding period will allow the ewe three opportunities to become pregnant.

Gestation: Average gestation length or length of pregnancy for sheep varies from 144 to 151 days (about 147 days; figure 11). Individual pregnancies may vary from 138 to 159 days. Generally the earlier maturing meat breeds and the high prolific breeds such as the Finn have shorter gestation lengths (144 to 145 days), while the slower maturing finewool breeds (Rambouillet) have longer periods (150 to 151). Typically multiple lamb baring ewes will have slightly shorter gestation periods. It is not unusual for individual ewes within breeds to vary in gestation length by 3 to 5 days.

 

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