Testicle Traits
By Dr Mackie Hobson BSc(Agric),BVSc

Thursday, 26th April 2018

Why is testicle size important?

  1. Sperm production

Research has proved that sperm production is correlated to testicle size, testicle weight and scrotal circumference. (Lino 1972, Ott&Memon 1980, Langford et al 1989)  (Gebre 2007)

Each gram (g) of testicular mass produces 25 million sperm per day

  • An effective ejaculation of 0.5ml of good quality semen will produce in the region of 1,500 million sperm

The importance of testicle size/weight can be demonstrated in the two examples below. How many ewes can a ram cover? Keep in mind that on average that each ewe in heat is mated about 3 times).

  1. Increased libio
  2. Increased fertility of offspring
  • Increased ovarian activity in female offspring
  • Daughters reach puberty at earlier age

What other factors affect the quality and quantity of sperm?

  1. Age (Toe et al 1994. Al-Ghalban et al 2004)


  1. Body Weight and Condition (Alkass et al 1982, Toe et al 1994, Schulte-Hostedde et al 2005)(Rajuanal, Tyabur et al)

Testes measurements were strongly associated with age, body condition score and especially with body weights.

Bucks’ age, body weight and body condition score were found to be positively correlated with ejaculate volume, semen density, total sperm (A. K. Rajuana, M. R. Tayabur, M. A. Hoque, S. S. Husain and Z. Sultana) and percentage of normal sperm, which were in agreement with the findings of Colas (1976).



  1. Season (Greyling &Grobler 1983, Al-Ghalban et al 2004)


  1. Nutrition
  • Sperm production and number per ejaculate increased with better nutrition (Martin et al 1994, Thwaites 1995, Almeida et al 2007).

Energy and Protein in diet (Parker &Thwaites 1972, Braden et al 1974, Foote et al 1978, Oldham et al  1978, Alkass et al 1982, Fernandez et al 2004, Kheragmand et al 2006)

  • Proportion live versus dead sperm per ejaculate

Not effected by diet (Kheradmand et al 2006)

Energy intake and reproductive performance (Braden et al 1974, Rowe and Murray 1984, Murray et al 1990)


  • Protein deficient diet decreases testicular growth and spermatogenesis (Oldham et al 1978)
  • Protein deficient diet decreases reproductive performance. (Martin et al 1994)



  • Protein above maintenance effects unclear with an increase or no changes in teste size, semen quality, sexual activity (Oldham et al 1978, Lindsay et al 1984, martin et al 1994, Boukhlig et al 1997, Fernandez et al 2004)


Is it justified selecting for testicle size?

When we want to select for a particular trait in the Angora goat we need to know the heritability of the trait to determine the value of selecting for this trait.

The heritability of testicle size in the Angora goat has not been determined but it has been proven in sheep breeds to be high: 0.35 in sheep (Sheep Production 2002 edition, Volume 7)

How does a heritability of 0.35 (35%) for testicle traits compare to other traits of known heritability in Angora goats.


Heritability in the Angora goat

Greasy Fleece Weight


Fibre Diameter


Birth Weight


Weaning weight


16 month body weight










Worm resistance*


Data from Grootfontein research (Snyman/Visser/marle-koster),(M.Snyman),(M.Snyman,Olivier)

*Sheep value


Is our current selection for testicle size adequate?

There are no current significant problems with fertility of our Angora goat rams but we must avoid introducing a fertility problem over the coming decades.

How do we currently select for testicle traits?

  • Testicles are palpated and checked when the rams are classed.

Although there is no objective measurement (no breed standard) the classing is done on experience and relative comparison to other rams in the group.


  • Are we not eliminating rams with small testicles when we have strict selection criteria of weight? (Knowing weight and testicle size is correlated). So no need to worry?


To answer these questions we have had a look at Angora Ram testicular data collected over the last 3 years (2015-2018).


Scrotal circumference


The different colours represent the Graaff-Reinet and Jansenville Veld Ram testicle circumference over the different years in the graph below.

Combined data (n=609)



 The effect of nutrition on scrotal size (graph below)

The blue line of the fed ram sales can be compared to the testicle size of the veld ram sale.

With any population data curve we expect to see the normal bell shaped with the standard deviation from the median as in the graph below.

With added nutrition as in the fed ram sales in the previous graph we get:

  • a shift of the curve to the right but
  • there also appear to be a slight Negative skew graph.

What can we conclude from this?

  • Nutrition as is well known increases testicle size
  • The graph tends towards a phenotypicall maximum sized testicle for this age of ram (negative skew).
  • Nutrition will mask gentically inferior rams (fertility).

Culling rams on weight at final selection - are we not removing the smallest tesicles anyway?

To answer this we have to look at what has happened over the last 3 years.

If for example we decided that as a ram buyer we would not want to buy a ram with testicles smaller than 23 cm. That is the bottom 1-7% of all rams (n=609).

If we culled on weight how many of these rams with small testicles would not get onto the sale anyway? -so not of any concern?


Veld Rams

Rams culled below (Kg)

% testicles



















none culled on wt










From this we can see that on average only 15.12% of the smallest testicles are being removed when the rams were culled on weight. On average 84.88% of the rams with the smallest testicles are getting onto the sales.

  • The less fertile rams are not being culled when selected on weight

This is happening due to the HIGH heritability of testicle trailts.


What is the future of testicle measurements?

It has been proved that each gram/volume of testicle increases the number of sperm produced.

Testicle traits are highly heritable. Therefore selection for scrotal circumference may over many decades lead to ‘round testicles’ where ultimately it is volume of testicle that we need be selecting for.

For this reason Dr Johan Van Rooyen at Grootfontein Agricultural college worked out the most accurate way of determining testicular volume (using cylinder and sphere clculations). It is easy eanough to measure the scrotal circumference and lenth and plug these values into the formula to get the testicular volume.

Example below comparing 2 rams for testicular volume.

  • Ram with a circumference of 30cm and length of 11cm: Volume =507 cubic cm
  • Ram                                            22cm                            8cm                 = 199


By plotting the circumferences and lengths and formulating a table for Angora goats.


  • It is therfore easy to set a minimum ‘Volume’ as a ‘standard’.


Split scrotums are a highly heritable trait. So much so that when collecting the data over the last 3 years that I could with >90% certainty say which studs some the goat rams originated from just by checking the scrotum.

Split scrotums are undesirable because of the:

  • Link to split udders in does.
  • Risk of added trauma, thorns getting caught in the split and a crevice for ticks.

There has however been NO scientific paper written to suggest a split scrotum causes any reduction in fertility.  Most sheep breeds as well as the USA Angora goat have a breed standard which varies from under 3cm to having a split of less than 1/3 the length of the testicle.

The different colours represent the different years of the veld rams in the graph below.

Combined data: Number Veld Rams with different scrotal splits



With testicular traits being highly heritable SAMGA needs to consider long term strategies to avoid reproductive issues creeping into the Angora goat industry.


Dr Mackie Hobson


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