Genetics
In recent years there has been a rapid increase in genetic merit in UK dairy herds. This rapid increase has largely been attributed to the importation of semen from North America and Europe, coupled with advances in progeny testing schemes and the introduction of advanced techniques to evaluate progeny test data from different countries.
Assessing Genetic Merit
A genetic index is a measure of an animal’s ability to transmit its genes to the next generation. For dairy bulls genetic indexes are calculated from the performance of daughters (progeny tests) with other factors such as the genetic index of other relatives (particularly his sire) taken into account.. For dairy cows a similar procedure is used, with the animal’s own performance being an important component.
Therefore genetic indexes are based on:
- The animal’s own performance (if available)
- The performance of progeny
- Genetic index of sire
- Genetic index of dam
- Genetic indexes of all other known relatives
As more information becomes available for an animal, the importance of relatives information declines. Records are also adjusted to remove management and environment effects such as month of calving, age at calving, calving interval and differences in feed level between herds.
Typical production PTA reliabilities (from MDC Breeding+)
| Reliability | Comment | Information contributing to PTA |
|---|---|---|
| 10-29% | Extremely low | Some Pedigree Indexes – involving estimates from relatives. |
| 30-40% | Very low | Most Pedigree Indexes – involving estimates from parents with reasonable reliabilities. |
| 41-55% | Low | Some Pedigree Indexes. Usually the sire is well proven and the dam has very high reliability. Cows with one or two lactations. |
| 56-65% | Low to moderate | Bulls with officially published PTAs (minimum 50%). Cows with 3 lactations. |
| 66-75% | Moderate | General maximum for most cows (3 lactations +). May be exceeded if a cow has many ET daughters in the UK. |
| 76-90% | Moderate to high | Bulls with an initial progeny test through AI. |
| 91-98% | High | Proven bulls with a large number of daughters from a wide cross section of herds. |
| 99% | Very high | Widely proven and used AI bulls. |
Predicted Transmitting Ability (PTA)
Genetic indexes are expressed as PTAs, and these indicate the amount of milk, fat and protein or fat percent and protein percent which an animal is predicted to pass on to it’s progeny. A typical set of PTA values would be:
| Milk (KG) | Fat (KG) | Protein (KG) | Fat % | Protein % | Reliability % |
|---|---|---|---|---|---|
| +307 | +18.2 | +12.6 | -0.02 | -0.01 | 95 |
All PTAs are expressed in terms of deviations from a fixed genetic based. The current base is PTA2005. The base year heifer equivalent production for PTA2005 is
| Milk KG | 7538 |
| Fat KG | 286 |
| Protein KG | 243 |
| Fat % | 3.79 |
| Protein% | 3.22 |
BREEDING VALUE
The animals PTA is a measure of the genetic potential passed on to the animals progeny. The femle animals actual performance is indicated by its breeding value. The breeding value is twice the animals PTA figure.
How to use PTAs
To best demonstrate the use of PTAs it is necessary to consider a particular mating, and consider the effect on milk yield.
MILK YIELD (PTA) | ||
SIRE + 307 | DAM + 131 |
|
PROGENY (heifer) + 438 (Breeding Value) + 219 (PTA) |
In this case the heifer would be expected to yield 438 KG more milk than the genetic base, and would be expected to pass on 219 KG milk to her progeny.
Progeny Breeding Value | + | Base Heifer | = | Progeny Predicted |
|
(Sire PTA + Dam PTA) | Equivalent Production | Heifer Yield (KG) |
|||
Milk (KG) | 307 + 131 | 7538 | 7976 |
||
Fat (KG) | 18.2 + 2.5 | + | 286 | = | 307 |
Protein (KG) | 12.6+3.9 | 243 | 260 |
PTAs are also available for somatic cell count (SCC), lifespan and fertility.
Somatic Cell count: PTA for SCC are published in the range + 30percent. Bulls with negative PTA, for example,. –10 percent would be expected to pass on a 10 percent reduction in cell count. Those bulls with positive PTA values for SCC would be expect on average to increase cell count.
Lifespan: Lifespan PTA allows selection for longevity within the herd. Lifespan PTAs are calculated from type and lactation data. Four major type scores are used in the calculation:
- Fore udder attachment
- Foot angle
- Udder depth
- Teat length.
Lifespan PTA has also been given an economic weighting based on replacement rate due to involuntary culling, milk price, cost of replacement, value of cull cow and feed costs.
Fertility: The PTA for fertility was launched in 2005 and is based on records in the national database relating to fertility traits (calving interval and interval to first service), insemination traits (non-return rate and inseminations per conception) and correlated traits (110-day milk yield and body condition score). The PTA for fertility ranges from -10 to +8. To help improve the future fertility performance of your herd it is important to select sires with a positive PTA for fertility.
Economic Indexes (£PIN or £PLI)
Profit Index or £PIN
This is an economic index calculated from milk, fat and protein PTAs. Each of the PTAs for milk, fat and protein are weighted according to:
- the prices paid for milk constituents after taking into account the costs of production, for example, feed energy and quota
- the costs of processing, storage and transport of milk.
£PIN is the expected difference in net margin per cow per lactation
The calculation for £PIN is:
£PIN = (-0.03 x Milk KG PTA) + (1.20 x Fat KG PTA) + (3.00 x Protein KG PTA)
Profitable Lifetime Index or £PLI
This is another economic index which includes weightings for production and longevity. The basic description would be £PIN plus a lifespan value which takes account of locomotion, udder, somatic cell count, fertility and general lifespan traits. The PLI index was revised for the August 2007 proof run placing much more emphasis on lifespan. Further details are available on the MDC Breeding+![[external site]](extlink.gif)
website.
website.£PLI is the expected difference in net margin per cow lifetime based on production and fitness traits. These now have a weighting of 45 percent on production and 55 percent on fitness traits.
For bulls with a locomotion score proof available the calculation for £PLI is:
£PLI = [(-0.027 x Milk KG PTA) + (0.80 x Fat KG PTA) + (1.71 x Protein KG PTA) + (25.4 x Lifespan PTA) + (-0.19 x SCC PTA) + (1.13 x Locomotion) + (1.81 x Udder) + Fertility Index] x 3.5
For bulls with only a foot and leg proof available the calculation for £PLI is:
£PLI = [(-0.027 x Milk KG PTA) + (0.80 x Fat KG PTA) + (1.71 x Protein KG PTA) + (25.4 x Lifespan PTA) + (-0.19 x SCC PTA) + (1.13 x F&L) + (1.81 x Udder) + Fertility Index] x 3.5
Up-to-date bull PTAs, £PIN and £PLI can be obtained from the MDC Breeding+ or Holstein UK websites.
or Holstein UK websites.