There is no doubt that fertility is the most important trait which should be selected for. A calf is the product that is sold, without a calf to sell, there will be no profit. Fertility is however a complex trait with a simple result: the cow either produces a calf or she doesn’t. The reasons for not calving are many, e.g. environmental, hormonal, and also, genetic. The heritability of fertility is generally low - around 10% or lower. This implies that, with present methods, only around 10% of genetic factors can be explained. It is a difficult trait to measure directly, and is generally indirectly measured with indicator traits, e.g. age at first calving, number of calves up to a certain age, scrotum circumference, and others. On a genetic level, fertility (a cow calves or not) is what is called a threshold trait.
This breeding value is an indication of fertility as well as the retention of a bull’s female progeny. It actually indicates the bull’s success in the breed. All daughters of a bull are classified as follows :
0: Culled before calving (indicating that the daughter was born, but not retained in national herd. Young heifers are usually retained for reasons other than fertility, e.g. she is likable, her sire was expensive, etc. Some daughters in this group will, however, be infertile)
1: Calved once before the age of 6 years (indicating that, although she was given a chance, she was not retained in the herd)
2: Two calves before the age of 6 years
3: Three calves before the age of 6 years
4: Four calves before the age of 6 years (Fertile daughters)
These classification measurements of a bull’s daughters are used to calculate breeding values for the bull. In order for a bull to receive a high breeding value for calving tempo his daughters must be retained in the herd as a young heifer, and must regularly calve up to 6 years of age. Bulls with low breeding values do not necessarily have fewer daughters, but they do have poor retention in the herd. This breeding value reflects the number of calves that 100 of a specific bull’s daughters will give above or below the average. The measurements of all related animals are taken into account through the pedigree information available. Currently this breeding value is only available for males in the breed.
Stayability is only estimated for the SA Angus. It is interpreted as the number of cows still in a herd by the age of 6 years.
Scrotum circumference is an indication of both fertility of the bull itself, as well as fertility in his daughters. Scrotum circumference is an intermediate trait, meaning that both extremes are undesirable. A small circumference indicates low fertility, while too high could indicate high fat deposits, which could lead to infertility. This trait is measured in young bulls in post wean tests (Phase C and D), while females and untested bulls receive breeding values based on pedigree information.
Birth weight breeding values are very important, especially in certain breeds. Birth weight is highly correlated with weights at later ages, meaning selection for growth traits will increase birth weight as well. High birth weights are one of the major causes of calving problems, which cause loss of calves, loss of cows and lower further fertility of cows that had difficult births. It is however possible to increase weaning weight, but to select against higher birth weight. Birth weight has, like weaning weight, a direct and maternal component. A calf has its own genes which determines its birth weight, and is indicated by the direct breeding value (EBVbirth-direct). The growth of the fetus is however also influenced by its dam’s genetic ability to restrict its growth in the womb. This trait is indicated by the maternal breeding value (EBVbirth-maternal). It is important to realise that the direct breeding value is based on the animals own measurement, while the maternal breeding value is ‘measured’ in the progeny. Direct and maternal breeding values are described in more detail under weaning weight.
Growth rate traits
The growth of a calf is influenced by the environmental effects on the cow and her calf, the genetic composition of the dam to look after the calf and to feed it as well as the genetic composition of the calf for growth ability. The dam determines, to a large extent, the environment of her calf from conception to weaning age. In the case of weaning weight, the calf has a genetic ability for growth (indicated by the direct breeding value) and that the dam of the calf has a genetic ability to create an environment (mainly milk production) for the calf to grow in (indicated by the maternal breeding value).
- Yearling and 18 month weight
These weights are measured mainly on heifers in Phase B, as bull calves are generally measured in Phase C or D after weaning. These EBVs reflect the differences between animals for total growth ability up to one year and 18 months of age respectively. The EBVs reflect not only the growth ability of an animal, but also to a degree, the mature weight of an animal. Under extensive farming conditions, cows of an average weight do better than large animals. Select for average EBVs.
This breeding value is estimated for breeds that have enough cow weights on the system. Weights of cows 4 years or older, that were weighed at the weaning of their calves, are used in the estimation of the breeding values. Select for average EBVs.
ADG EBVs reflect genetic differences between animals for post-weaning growth. It is measured during Phase C and D growth tests. Select high breeding values for maximum growth, if farming under extensive conditions.
Feed conversion ratio is the amount of feed a bull needs to increase 1 kg in weight. It is only measured in Phase C as each bull’s individual intake needs to be measured. EBVs reflect the genetic differences between animals to efficiently convert feed to body weight. Animals with low feed conversion ratio EBVs are more efficient.
In cases where it is impossible to measure individual feed intake (e.g. Phase D tests), Kleiber ratio gives an indication of the animal’s feed/veld usage. The Kleiber ratio can thus serve as an indirect indication of feed conversion efficiency. Animals with a higher EBV are more efficient.
Feed intake breeding values are estimated by taking into account the bull’s intake in Phase C. Feed conversion is the ratio between intake and gain (the quantity of feed necessary in order to gain 1 kg of mass). By publishing intake breeding values, it has become easier for breeders to identify more efficient animals. Although the bulls don’t differ significantly with regard to ADG, they do differ significantly with regard to Feed Conversion and Intake.
- Feedlot Profit Index (FPI)
Selection Index composed from EBVs calculated for intake, begin weight and end weight. This EBV reflects the profitability of a bull in a feedlot or equivalent environment.
Body measurement traits
- Shoulder height and body length
EBVs reflect the genetic differences between animals for shoulder height and body length, as measured in Phases C and D growth tests. Generally breeders prefer longer animals (more capacity for meat), but not too high at the shoulder.
Meat quality (RTU) traits
Meat quality traits are measured on live animals with sonar or ‘Real-Time-Ultrasound’ (RTU). Breeding values in South Africa are only available for Angus cattle, but will be available to other breeds as soon as enough measurements are available. Measured traits include fat thickness, eye muscle area and marbling (intramuscular fat). Marker genes for meat tenderness have also been identified recently.
Breeding value indices are the easiest way to select on breeding values! The breed index compares animals (as in the case of EBVs) across the breed. This value indicates the percentage deviation in genetic ability of a specific animal when compared to the mean of the measured living animals in the breed, where the average compares to 100. The group index indicates each animal’s percentage EBV deviation from the mean of his/her group. Only animals with measurements for the trait concerned qualify for such an index. Animals can be compared directly to each other using the breed index or compared to their contemporary group using the group index. These indices are similar in expression to the well-known indices of the National Beef Cattle Scheme. The primary difference being that these new values are based on BLUP breeding values and thus take all environmental factors into account.
Commercial breeders interested in buying bulls from more than one breed can use the breed index to determine to what degree a bull is better or worse than the breed average (of living animals) without even knowing what the breed average is. Although the methods whereby these indices are calculated were developed locally, similar indices are already being used elsewhere in the world.