Ken Wilson, formerly of the Agency for Food and Fibre
Revised by Nicole McLennan, Agency for Food and Fibre Sciences
- Viable sperm production takes 49 days so rams need to be well looked after during the two month period leading up to joining to avoid factors that adversely affect sperm production.
- Palpation of the epididymis tail is a good guide to level of sperm reserves; tail should be large and firm.
- Testicle size is a good indication of sperm producing ability and more emphasis should be placed on it when selecting rams.
- Supplement with protein and energy for two months prior to joining and aim to have rams at fat score of 3.5 at joining. Over fat rams are less sexually active and more prone to heat stress.
- Heat stress can cause temporary infertility - lack of paddock shade, mustering, flystrike or infection.
- Shear rams so they are carrying three to four months wool at joining.
Process of sperm production
The process of sperm production in the testes is called spermatogenesis and each tadpole shaped sperm is a spermatozoa. Although only one spermatozoa is all that is required to fertilise each egg produced by the ewe, there are several million sperm ejaculated at each service to improve the chances of the egg being fertilised.
Primary sperm cells are produced by testicular tissue in the body of the testicle (Diagram 1) and undergo several stages of development before passing out of the testis and into the epididymis. The process of sperm development takes 35 days to complete.
The fertility of sperm when it first enters the head of the epididymis is very poor. Maturity and maximum fertility is reached over a 14 day period during which sperm cells move through the body of the epididymis.
The entire process of viable sperm production takes 49 days. Hence the need to ensure rams are well looked after during the seven to eight week period leading up to joining to avoid factors that could adversely affect the sperm and lead to temporary infertility.
Diagram 1. The testicle of a ram (right hand side when viewed from behind).
The epididymis absorbs testicular fluid and concentrates the spermatozoa into a tightly packed mass enabling large quantities of sperm to be stored in the smallest possible space. Healthy, well-fed and sexually rested rams may hold up to 100,000 million sperm, of which about 75 per cent will be stored in the tail of the epididymis.
Palpation of the tail of the epididymis is often a useful guide to the level of sperm reserves in individual rams. A large, firm (but not hard and diseased) tail is an indication of good reserves whereas a small, soft tail would indicate the opposite.
Sperm are produced by testicular tissue at a reasonably constant rate of about 20 million sperm per gram of testis per day (28 grams = 1 ounce). Testicle size is therefore a good indication of the sperm producing ability of individual rams.
Not enough emphasis is placed on testicle size at either the stud or commercial breeding level and although it is not necessary to select for excessively large testicles, those on the smaller end of the scale should be avoided.
The male hormone testosterone is also produced in the testes and is essential for the development of male characteristics, for the maintenance of active sexual behaviour and to support the process of sperm production.
Nutrition has a direct and dramatic effect on testicle size, which has a corresponding effect on sperm production. Rams grazing pastures of fluctuating quality may have testes which double (or halve) in size during the year due to the seasonal quality of pasture. In fact, research has shown that an improvement in nutritional intake of both protein and energy during the two-month period prior to joining can increase testicle size and subsequent sperm production by as much as 100 per cent. Aim to have rams at a fat score of 3.5 at joining. Also, nutritional changes affect testicle size much more rapidly than is reflected in liveweight or general body condition, which highlights the importance of checking the rams' reproductive soundness prior to joining.
On the other hand, rams should not be allowed to become over-fat (condition score greater than four) as over-fat rams tend to be less sexually active and are more prone to heat stress.
Vitamin A is required for sperm production. Rams deficient in vitamin A have soft testicles and produce poor quality sperm. Where rams have gone for prolonged periods of time (ie. six months or more) without access to any green feed, supplements which contain Vitamin A may be required (eg. green hay, vitamin concentrate). Mature sheep have sufficient stores of vitamin A to survive for eight to twelve months without green feed and without showing signs of vitamin A deficiency. So vitamin A deficiency is not normally a problem, as the rams will generally have some access to green feed during the year eg browse, pick after storms etc.
For normal sperm production to occur it has to be at a temperature several degrees below that of normal body temperature, otherwise sperm production may be affected. To provide the necessary cooling mechanism, the ram has large sweat glands in the skin of the scrotum as well as a system of muscles that raise or lower the testes into the body for the purpose of temperature regulation. Blood flow to the testes also helps to regulate temperature through a heat exchange mechanism. Heat is transferred from the testes to the blood and is transported to other parts of the body for dissipation.
If the temperature in the testes cannot be kept low enough, as can happen in hot weather (eg temperatures over 32 degrees Celsius for long periods or short spells of very high temperatures (38 degrees or more)), the production of viable sperm will be affected. Fully developed stores of spermatozoa are less affected than are sperm in the developing stages. This may mean that rams that are heat stressed before joining could still successfully serve ewes up until such time as the sperm stored prior to the heating are used (say two to three weeks). However, the seven week delay in viable sperm production will then commence, as this is how long it takes to produce new viable sperm.
Some strains of Merino may be more heat tolerant than others, but this may be due more to environmental adaptation than to any specific genetic differences. However, rams with excessive body wrinkle are much less able to cope with high temperatures than are plain-bodied rams as they have a poorer ability to control testicular temperature.
Another important consideration is when to shear in relation to joining. Shearing immediately prior to joining is not desirable, since some fleece protects the ram from heat. On the other hand a heavy fleece is likely to reduce mating activity and may impair sweating and cooling of the testes. The best plan seems to be to arrange shearing so that when the rams are first joined they are carrying three to four months' wool.
Temporary infertility from overheating can be caused by:
- lack of paddock shade
- mustering rams
- fever resulting from flystrike or infection.
So all efforts should be made to protect the rams from these conditions prior to and during joining.
The Lamb Boost Project funded by Australian Wool Innovation produced these notes.
For further information contact the DPI Customer Service Centre on 13 25 23 (Queensland residents) or (07) 3404 6999 (non-Queensland residents) between 8 am and 6 pm weekdays, or e-mail firstname.lastname@example.org.
This DPI Note is also published on the DPI's PrimeNotes CD-ROM.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
File No: SW0085 . Date created: February 2002 . Reviewed: February 2003