Fertility and Fat

Fertility, Fat, Fads and All That!

Each year at about this time there are articles circulating research findings documented and plenty of discussion generated about topical issues re cattle breeding. More recently there appears to have been a focus on Fat and Fertility, it seems the fashionable method to improve female herd fertility is to increase total fatness.

The logical question that should follow re increasing fatness so as to improve female herd fertility is:-

Are there any problems that may impede herd productivity associated with this increasing fatness?

Prior to launching into a discussion re the question above it is important to understand why fat is such an essential tissue within the bovine.

What is Fat? Why is it important?

Fat is a reasonably dense storage molecule which, under times of stress in the living animal is mobilized and converted to ‘energy’. This released energy is used to drive normal body functions.

Fats other functions include:-

a)            Protecting carcases from microbial spoilage

b)            Protecting the muscle during refrigeration from cold shortening

c)            Driving the juiciness flavour sensation

When analysing fatness it’s a not case of “if a little bits good, then a lots got to be better”, surplus fat does impede cow herd productivity.


  • Reduced fertility
  • Increased calving difficulties
  • Reduced milking ability
  • Reduced C.F.A cows and non- pregnant heifer dollar value

In addition surplus fat reduces male fertility. When fat is produced to excessive amounts in the male fat will become deposited around the scrotum. Because fat is an insulator, surplus amounts will prevent the scrotum maintaining a temperature below normal body temperature thereby inhibiting acceptable sperm production. Further, surplus fat restricts the mobility and longevity of bulls and so their ability to effectively serve 50 to 60 cows per year for five seasons.

Finally surplus fat impacts heavily on saleable beef yield. Those extra millimetres of subcutaneous fat over and above the required maximum of 17mm at the p8 site, cost the beef industry major dollars, by raising the production costs from having to trim the fat in the abattoirs and at retail. Further it takes 2.25 times as much energy to produce a kilogram of fat as it does a kilogram of muscle.

Some fat is essential, surplus fat creates problems.

How best to improve and maintain fertility!

If increasing total fatness is likely to result in problems such as those discussed then it’s obviously not all that desirable a method of improving fertility.

Fertility is a quantitative trait, it is able to be measured and it has an impact on dollar value and outputs from a ‘whole herd’. The phenotypic variation for the quantitative traits is a result of the combination of:-

  • Genetic make-up or heredity
  • The environment
  • The combined action of heredity and the environment.

Heritability estimates indicate the amount of total phenotypic variation caused by the genetic make-up (heredity). The heredity estimates are expressed as percentages. The heritability estimates recorded in the literature for fertility are low and range from 0-15. This means that by far the largest contributor to the expression of fertility rates is the environment that is, disease, nutrition, management etc.

Improvement for any quantitative trait can only be achieved by knowing the base line performance and then working on both the heritable components (genetic) and the environment. Measuring baseline fertility performance in the male is done by:-

  •  Measuring scrotal circumference in centimetres
  •  Examining semen- live sperm cells, progressive motility, abnormalities and density.

While scrotal circumference measurement is repeatable, semen quality tests are influenced by many variables (stage in cycle, nutrition, temperature at which testing occurs etc.). As a result of fertility being lowly heritable, most of the variation is accounted for by the environment. Measuring base line fertility in the cow herd is done by;

  •   Ensuring a restricted joining interval eg. Heifers 6 weeks and a couple of days.
  •  Pregnancy checking and calculating percentage pregnant.

The check list in managing the cow herd for high pregnancy rates should include:-

i.            Reproductive disease control particularly Lepto Spirosis, Vibrio and Trich. Vibrio and Trich  are transmitted by the sire and are best controlled by managing the sires. Other disease control/prevention e.g. pesté virus etc.

ii.            Managing nutrition so that maiden heifers (13-14months) achieve joining weights of 300-320kg. Also ensure heifers and cows are on a rising plane of nutrition just prior and during joining. When soils/pastures are known to be deficient in zinc, copper and selenium then it may be beneficial to administer these trace elements as injections.

iii.            Manage the parasite burden, internal and external.

iv.            In the more favoured environments develop a breeding strategy- best time to join, how long does the joining interval need be, how many bulls per 100 cows etc.

v.            Carry out pregnancy checks and remove the empty females. On today’s market when cows are worth $3.00kg HSCW and 6 and 8 tooth bullocks sell for $2.80- $3.00kg HSCW it seems ludicrous hanging onto empty breeders.


The check list for managing the sire battery for maximum fertility includes:-

i.            Ensuring scrotal circumference is in excess of 36cm for 2 year olds and 32cm for yearlings. Preferred scrotal shape, tone, length of neck etc.

ii.            Administering  Vibrio and Lepto injections

iii.            Controlling internal/external parasites

iv.            Organising nutrition so that bulls are on a rising plane for a period prior to and for the duration of the joining period. Ensure bulls aren’t too fat or too lean and that they are sound. Where trace minerals aren’t abundant in soil/pasture may have to administer, e.g. selenium

v.            Match bull joining to cow preg check performance.


Genetic Correlations – Where do They Fit When Evaluating Fertility?

Genetic correlations definitely exist. Their use in breeding programs can be beneficial, of academic interest only and/or misleading. One correlation that is beneficial and of great benefit to maternal progress, is that of relating scrotal circumference to per cent first oestrus conception in maiden heifers. To correlate within a trait in this case fertility is a very useful exercise. First oestrus conception is a major profit driver in the beef herd. Maiden heifers that settle in calf ‘late’ in the joining period will have less time to build their body reserves to cope with the fact they are:-

  • Still growing
  • Lactating
  • Needing to ‘cycle’ within three months of calving to settle in calf as a two year old.

The heifer’s nutritional requirements to fulfil the above list of functions, is greater than at any other 3 to 4 month time interval of her life. Don’t lose sight of scrotal circumference as a fertility selection criteria the ideal goal being 38-42cm.

The consequences of breeding for fatness to improve fertility.

The heritability estimate for increased fatness at the 12th/13th rib is 40-50. That means the expression of fatness is far more dependent on heredity then is fertility (heritability estimate 0-15). Therefore selection for fatness will result in much greater levels of fatness being achieved given similar environmental conditions. Along the way this pathway of selection for increased fatness may result in a few more first oestrus  conceptions within the 6 weeks breeding interval, it may well also result in more two year olds settling in calf. However soon enough because of the high heritability the time will come when this increasing level of fatness will reach a point where it will be counterproductive on female herd output (cf. earlier discussion on the role of fat and the problems associated with excess fat). These excessive fat levels could conceivably be reached in as few as two generations if too great an emphasis is brought to bear on the trait. The consequences of such a selection process may not only result in reduced fertility but also change the maturity pattern of the herd to the point where kilograms of beef are reduced. Remember the growth progression of bovines. The first tissue to grow and develop is Bone, next is Muscle, the final tissue is fat. By placing too much pressure on the third or final tissue to grow and develop, the development phases of bone and muscle are reduced to the point where lack of skeleton plus muscular growth and development results in smaller lighter animals. That is, early maturity will be promoted by an over emphasis on fat development and will result in reduced beef herd productivity and so profitability.

Can growth have a negative impact on fertility?

The bovine body mass is composed of three tissues, bone, muscle and fat. These tissues grow and develop in that order. Growth is measured in kilograms and growth rate, the more meaningful measure is measured in kilograms per day. Any increases in growth/ growth rate are the result of increases in the development of the three tissues; bone, muscle, fat. As a general rule once sexual maturity is reached, bone development is complete and muscle development almost complete. Therefore any increase in growth/ growth rate post puberty is the result of some further muscle development but mostly it is the fat adding to the increase in mass.

The phenomena of first oestrus occurring during the later stages of muscle development means puberty will be delayed by placing too much emphasis in selection for growth. That is, the time animals spend in the bone and muscle growth development phase is extended. It maybe that the heifer won’t slow in muscle development until 17-18 months, therefore she won’t be able to settle in calf until 17 – 18 months  or 3-4 months post the preferred chronological age to suit the management of female herd fertility. Conversely by placing too much pressure on selecting for fatness the time the animal spends in the bone and muscle development stage is reduced, the heifer reaches puberty very early (9-10 month) thereby causing problems managing female herd outcomes. This will also restrict growth rates and therefore slaughter weights with respectable fat covers to below 280kg HSCW.

The Dulverton Experience

Since the beginning of the Dulverton herd our goal has been to improve growth rate yet not at the expense of fertility. Fertility after all, is, as Jim Bradford so aptly described it the ‘governor of growth’ (see article on Fertility on Chaps Chatter). In 1981 when we started the Dulverton Stud the Japanese were in Australia sourcing Angus for their meat quality attributes, unfortunately though the mature weights of Angus prevented them from achieving the 400 plus kg carcase weight specification. So for the past thirty one years we have selected for fertility and growth. Interestingly improved herd performance has come as a result of both traits being improved simultaneously.

When we commenced, our maiden heifer joining weight was 280kg, today it is 330kg. Interestingly this increase resulted in a reduction in 1st calf heifers calving difficulties primarily because the heifers are now 460/470kg minimum when they calve. That is, the skeleton and therefore the pelvis is more developed, that is, more room for the birth process to occur unassisted.

This year some thirty years on we have made a very interesting observation. In October 2011 we joined 93 purebred Angus heifers. At scanning on the 27th February 2012, 14 of these heifers had recorded growth rates of 0.89kg/day the range for the 93 was 0.75kg/day to 0.96kg/day. At pregnancy testing just prior to scanning 9 heifers registered empty. The 9 empty heifers all came from the sub set of 14 heifers with growth rates of 0.89kg/day to 0.96kg/day. Importantly we did achieve our pregnancy rate goal for the maidens of 88%, with this years result being 90%. The joining interval is 6 weeks and 4 days. However a major warning signal has been sounded and that is we now have enough growth for our breed and environment.


The following table details the heifer growth and fat performance for the 84 pregnant and 9 empty purebreds.

P8 fat mmRib fat mmGrowth kg/day
84 pregnant average11.27.40.86
9 empty average9.56.70.91
84 pregnant range5- 255- 140.75- 0.96
9 empty range7- 135- 100.89- 0.96




Points of interest;

  • The leanest heifer settled in calf
  • The fattest heifer settled in calf
  • The empties have grown 0.15kg/day more than the pregnants, to 550 days and at that point they are only slightly leaner.

The take home lesson for us is that given our environment we now have sufficient growth. It will be important to monitor the composition of the growth and be sure that growth rate of about 0.75 to 0.95 isn’t composed of any more fat then we currently witness.


  • Fertility is a trait of major importance. Fertility best expressed as the percentage of females settling in calf post a joining interval of a specified time.
  • Fat is a concentrated storage molecule with many important roles to fulfil in the bovine. Fat is the last of the three tissues of the bovine (bone, muscle and fat) to grow and develop. Must get fat contents ‘JUST RIGHT’.
  • Traits exhibiting low heritabilities e.g. fertility best improved by adjusting/ treating environmental influences.
  • Fertility maybe a trait of low heritability but infertility is highly heritable. Result bulls with scrotal circumference (less than 36cm), will breed too many replacement heifers that are slow to conceive. ‘Lazy’ cows (those that calve every second or third year), will past this trait on to their progeny with high frequency.
  • Fatness is moderately heritable, heritability and the environment about even in their contribution to the overall expression of fatness.
  • Fat best looked upon as having upper and lower limits. The concept of the abattoir generated Grid has its place in the cow herd. Too much fat reduces productivity, so to, too little fat.
  • Need to understand growth and its related traits (maturity pattern) to fully comprehend the negative impact a policy of selecting to increase total fatness to improve fertility will have on whole herd productivity.
  • The environment is a great communicator it should be ‘listened to’ to assist with the managerial decisions i.e. limiting traits such as growth to optimize the traits of low heritability e.g. fertility.
  • Long term whole herd productivity may well be reduced by attempting to improve fertility (a lowly heritable trait) through increasing female herd fatness (a moderate to highly heritable trait).
  • To improve female herd fertility don’t mess with fat, simply

– use bulls with scrotal circumferences in the range of 38 – 42cm,

– prescribe breeding intervals eg. heifers 6 weeks and cows 9 weeks

– set pregnancy rate target.

Posted in : Chap's Chatter