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This article was published in 1958
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Factors Affecting Fertility of Rams and Ewes and Survival of New-born Lambs

V. G. COLE, B.V.Sc., Veterinary Surgeon, Grazcos, Sydney

Though the Veterinarian is trained to be an expert in the treatment and prevention of diseases in animals, he also needs a working knowledge of animal husbandry. The following is a summary of the sort of working knowledge he needs in dealing with the important problem of fertility of sheep.


1. Nutrition—Rams must be in perfect physical condition at joining; which means that supplementary feeding may often be necessary.

2. Green Feed—Lack of green feed lowers fertility, due to Vitamin A deficiency. If rams are on dry feed two months before joining, experimental work has shown that dosing with one million units of Vitamin A is good insurance against possible infertility.

3. Heat—Prolonged temperatures over 90°F. lower fertility. This is of most importance in Queensland and in western districts of New South Wales, such as Nyngan and west. In these areas, mating between December and March should be avoided if possible.

4. Light—Rams taken from Sydney Ram Sales in June to Queensland (into increasing hours of daylight) often will not work in the following spring. The Queensland Department of Agriculture showed that if they were shut up each afternoon in a dark shed a month before mating, they would work.

5. General Health—Prior to mating, rams should be free of worms, footrot, foot abscess and other diseases likely to affect their general health.

6. Diseases of the Testicles—Epididymitis is the most important disease. Various surveys have shown an incidence of about 6% in Merinos. The incidence in British breeds is higher. New Zealand workers have shown that Brucella ovis is the cause and they have developed a vaccine to prevent it. There is not enough evidence yet to indicate whether epididymitis in Merinos is due mainly to the same infection.

7. Breed—Merinos will mate at any time of the year. British breeds show a decline in interest in the spring and summer.


1. Ewes with Muffle Faces—Figures for Rambouillets in America, Romneys in New Zealand and Merinos in N.S.W., show that muffle-faced ewes produce fewer lambs than open-faced ewes (Rambouillets 10% less, Merinos 15% less and Romneys 15% less).

2. Dry Ewes—There are usually about 5% of dry ewes after the second mating in a normally fertile flock. If these ewes are culled out, the lambing percentage rises less than 2%. If ewes failing to raise a lamb are culled also, the percentage rises by about 4%.

3. Bad Udders—The number of ewes affected on one or both sides of the udder varies in different flocks. It may reach 14%. If they are culled out, figures from Chiswick in the New England show that the lambing percentage rises less than 1%.

4. Immaturity—New Zealand Romney breeders have found that stunted maiden ewes have poorly developed breeding organs and will not breed successfully until they are two and a half years' old.


British breeds have a well defined cycle-Romneys will not mate readily before mid-February—Merinos have a less well defined cycle. Low sexual activity occurs in September, October and November; and rises in December. Reaches a peak from February to June and then declines to December.


Two factors are important—the presence of the ram and decreasing hours of daylight. If ewes run continually with rams they go right off heat from September to December and their cycle reaches a peak from March to June. If they are kept away from rams except for a normal joining period of six weeks, at least 70% will mate in the September to December period after a lag of approximately a fortnight.

The effect of light is seen in cold countries such as Britain. Breeding starts several weeks after the longest day, so that lambs are born in the spring after the snow melts. An experiment conducted in Queensland by C.S.I.R.O. showed that ewes' cycles could be reversed by reversing the normal light rhythm. It confirmed that decreasing hours of daylight increases breeding activity.


Good nutrition is important for both mating and lambing. If ewes are flushed for three weeks prior to mating and for three weeks during mating in the autumn it increases lambing percentages by 10 to 20%; due to an increase in the number of twins. It has less effect in the spring. There always are more twins from an autumn than spring mating, specially if the plane of nutrition rising.


Management of the ewe should aim at increasing her weight prior to mating, then holding it steady for the next three months, then increasing it again for the two months prior to lambing. It is important, however, not to "steam up" ewes too much prior to lambing. A middle course is best. Over-fat ewes have light lambs and have trouble lambing. In a South Australian experiment, ewes weighing 131 lbs. had 7 lb. lambs and ewes 119 lbs. had 9 lb. lambs. However, the fat ewes had 90% of lambs marked and cut 11½ lb. of wool and the light ewes had 88% of lambs and 10 lb. of wool.


The principles mentioned in the previous section are often difficult to fit in with normal pasture growth. Improved pastures are very superior to most native pastures but they all have one main growing period in the spring. Pastures are dry over the summer, they may get a slight boost from autumn rain and then stop growing again in the winter.

Pasture management must aim at providing adequate feed for lambing by either autumn saving of pasture for a spring lambing or supplementary feed (when the autumn break misses out) for an autumn lambing.


Undernourished ewes are liable to develop pregnancy toxaemia during the last six weeks of pregnancy. Though glycerine gave promise early, it appears that ewes on a gradually falling plane of nutrition do not respond.

The only answer to the problem at present is supplementary feeding.


High temperatures during pregnancy cause the birth of light lambs. C.S.I.R.O. has shown this experimentally. It explains why autumn lambs in Queensland average 5 lbs. and spring lambs 7 lbs. yet autumn is the best feed period. Excessive heat also causes the early death and absorption of foetuses.


Worms, blowflies and particularly foot abcess have more serious effects on pregnant ewes than on dry ewes.


Work in New Zealand has shown that the common cause is infection with Toxoplasma. Other causes are Brucella ovis, Vibrio fetus and Listeria. Even in New Zealand, abortions are responsible for only a small percentage of the losses of new-born lambs. In other words, most of the new-born losses occur after birth from other causes.


There is no evidence that ewes in lamb benefit from administration of Vitamin A under Australian conditions. Experimentally, Vitamin A deficiency is responsible for the birth of weak and dead lambs.


The ideal is to have good feed conditions at mating and also good feed conditions at lambing. This is, however, rarely possible, with the result that a compromise is necessary.

The following table adapted from a paper presented by Dr. Watson of C.S.I.R.O. to the Australian Society of Animal Production illustrates how to go about deciding the best time to mate for a particular district. Each month of the year should be written down in a column and the effect that mating in each month will have on ewes and their lambs should be noted down as shown:

Month of
Ewes likely
to mate
Likely to lamb
after mating %
Likely conditions for
Survival of Lambs
Growth of
Jan. All 90-95 Fair Fair
Feb. All 90-95 Fair Fair
Mar. All 90-95 Fair to poor Poor
April All 90-95 Fair to poor Poor
May All 80-95 Fair to poor Poor
June All 80-95 Good Fair to good
July Most to all 50-90 ?(ewes may be too fat) Fair to good
Aug. Few to all 50-90 ?(ewes may be too fat) Very good
Sept. Few to all 50-90 Good Very good
Oct. Few to all ? Good Very good
Nov. Few to all 85-95 Fair to good Good
Dec. Few to all 90-95 Fair to good Good

This table was prepared for the Western districts of Victoria and there is no column for the ram. In hot areas this column should be added. From the table, Dr. Watson decided that June in the Western districts of Victoria was the best month of the year to mate. In deciding this, he hastened to explain that he did not take into account economic or labour factors.

A number of experiments have been carried out showing the results of mating and lambing at different times of the year.

Results of a few of these are shown below:

(a) New England, N.S.W.—Results from "Shannon Vale", Glen Innes:

Type of
Mating % Lambs
% Weaned Remarks
Native April 67 26 Average 3 yrs.
Native June 47 36 Average 4 yrs.
Half Native
Half Improved
April 91 69 Average 9 yrs.
Half Native
Half Improved
June 87 76 Average 11 yrs.

Feed conditions are better in April than June, accounting for a higher percentage of ewes getting in lamb. However, feed conditions and weather better in November than September for lambing.






(b) Trangie, N.S.W.—Figures for Peppin Merinos of mixed ages (groups of 240-428).

Year Mating % Lambs Mothered % Weaned Remarks
1942 October 65 60 Good Year
1943 April 83 75 Good Year
1943 September 70 65 Average Year
1944 April 96 87 Drying Off
1945 April 102 87 Dry
1946 March 112 96 Drought
1946 May 81 67 Drought

(c) Tooradin North, Vice—Figures for C.S.I.R.O. Field Station for Merinos on improved pastures.

Mating % Ewes Lambing % Lambs Born Alive
April-May 92 98
August-September 77 78
December-January 93 85

(d) Kybyolite, South Australia—Southdown rams on Border Leicester X Merino ewes.

Mating % Lambs Dropped % Lambs Marked Remarks
Mid Dec. 113 107 Average
Late Jan. 136 126 4
Early March 165 148 Matings

Some of the factors causing losses of new-born lambs are as follows:

1. Undernourishment—Undernourishment during the last six weeks of pregnancy causes the birth of undersized lambs and a poor milk supply.

2. Overnourishment—Greatest losses of newborn lambs occur in the undersized lambs and very big lambs. Deaths of big lambs apparently are due to injury during birth.

3. Infection—Occasionally new-born lambs suffer from infections, such as Blackleg. These are not common causes of losses.

4. Adverse Weather—Excessive cold and heat kills lambs, particularly if under-sized.

5. Predators—Foxes are probably the most consistent killers of newborn lambs. Hawks and pigs also cause some losses. In Queensland as many as 1000 foxes are killed during the lambing season.

6. Subterranean Clover Disease—This causes deaths of lambs prior to birth and dystocia.

7. Copper and Cobalt Deficiency—This causes ataxia and deaths of new born lambs in districts where deficiences occur.

8. Miscellaneous—There are a number of miscellaneous causes of losses of new-born lambs, such as deformities, goitres and abnormal presentation.


(These figures are taken from surveys carried out by Departments of Agriculture in Queensland, S. Aust, and N. Zealand, and by the C.S.I.R.O.)

State Losses
Queensland 36 per cent. (Merinos)
Armidale, N.S.W. 1952–23 per cent. (Merinos)
1953–18 per cent.
1953—20 per cent. (no inspection during lambing)
Yarding each night:
1952-10 per cent.
1953–11 per cent.
Tooradin, Vic. 12 per cent. (Merinos)
Kybyolite, S.A. Singles: 3.5 per cent. (Crossbreds)
Twins: 10.2 per cent.
New Zealand Singles: 7.3 per cent. (Romneys)
Twins: 14.3 per cent.


The object of breeding sheep is to make a profit out of them. High lambing percentages help by allowing a good culling rate and providing surplus sheep for sale. A good percentage is 80. This allows up to 30% culling and replacement of ewes before they are too old.

Everything revolves around when to mate. The fat lamb breeder wants an early lamb because it brings the highest price. The Merino breeder usually wants to fit lambing in to suit his available labour. Hormones may help the fat lamb breeder achieve his objective if he mates early in the ewes' breeding cycle, but he still has to watch the feed position at lambing and for the ensuing three months.

The Merino breeder can help himself by taking each month of the year separately and, assuming he mated in that month, answer the following questions:

1. Will the rams be fertile?

2. Will the ewes mate?

3. What percentage is likely to get in lamb?

4. Will there be many twins?

5. What will feed and weather conditions be like at lambing?

6. What will feed be like during the suckling period?

7. What will feed be like at weaning and after?

8. Have I enough labour to crutch or shear before lambing and look after the ewes at lambing and carry out lamb marking?

It was suggested at the beginning that a veterinarian should have a working knowledge of animal husbandry. As far as fertility is concerned, an attempt has been made to give an indication of the sort of working knowledge necessary.

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