4-Cull and replace females

Implement a female culling and replacement policy to maintain best herd structure

• culling heifers and cows
• retention rate of heifers

Guidelines for culling heifers and cows

Cull as early as possible but at a convenient time, commonly at weaning of calves. Initial culling is for females empty at pregnancy diagnosis or that experienced calving difficulties at the previous calving. Ultrsound pregnancy testing allows earlier diagnosis of pregnancy to be achieved accurately and earlier tactical culling decisions.

If pregnant, then cull on physical factors such as unsound feet and legs, damaged or lost teeth, aged over 10 years (or required age structure of the breeding herd for desired rate of genetic progress), history of calving difficulty or inability to wean a calf.

To achieve the targeted rate of genetic progress and change in herd structure, a defined culling policy is needed for older cows.

Carefully consider how to achieve a balance between the number of older cows in their optimum productive years, say between their third to sixth gestation, and the influx of new generation heifers as replacements.

If the animal is required during a change in the age structure or rebuilding of the herd, re-mate and manage as a group separate from the main herd. The cost of culling all infertile cows (ie not pregnant after 45 days mating) in a program aimed at realigning calving pattern can be too high to complete in one year. Correcting an unsatisfactory spread in calving may need to be completed over three years (refer to 'Guidelines for reducing the spread of calving' in Procedure 2).

When infertile heifers and cows without calves are not culled, these non-pregnant animals consume pasture that can be productively used to grow stock and increase the throughput of saleable product. If retained, these culls should be grown to a saleable weight for a target market.

What to measure and when

• Presence of foetuses at pregnancy diagnosis.
• Fat deposition rates to avoid fatty udder syndrome.
• If pregnant, then physical factors such as structural soundness, teeth, age, ability to wean a calf and history of calving difficulty.

Guidelines for the retention rate of heifers

Select replacement heifers

Heifers intended as replacement breeders must be selected for their ability to:

• reach critical mating weight (CMW) on a desired joining date
• become pregnant
• deliver a live calf unassisted
• rear a calf to satisfactory weaning weight
• re-conceive within 82 days of calving to maintain a 365-day calving interval.

When determining the number of heifers to join, it is important to allow for culling of heifers that fail this screening test. If too few heifers are retained, the balance of the breeding herd (target size and desirable age structure) cannot be maintained from year to year. Another option may be to purchase pregnancy-tested cows in-calf, but the impact on the herd age structure and breeding objective would need to be carefully assessed along with the issue of biosecurity.

Calculate the number of replacement heifers required

When too many replacement heifers are retained, they consume more high quality pasture per calf born than older cows. Although heifers lower the average age of the herd, they are more labour-intensive at calving and wean lighter calves. However, excess heifers can become valuable sale animals and increase the ability of a beef business to access a range of target markets.

Assess the numbers of heifers retained for breeding in terms of age structure of the herd and long-term sustainability

Assess the effect on profitability of options for maintaining heifers to increase enterprise flexibility, such as selling grown heifers (see Module 1: Setting directions).

Determine culling rate

Culling rate is determined basically by the heifer retention rate. High culling rates are possible only when the heifer retention rate is exceeds 60–80%, which is only achieved with a high degree of reproductive efficiency. This can only happen when:

• short joining periods are achieved
• high numbers of heifer calves have enough growing days to achieve the critical mating weight
• adequate nutrition is available from weaning to joining
• adequate nutrition is available from joining to calving to minimise dystocia and facilitate return to oestrus in first-calf heifers and mature cows.

High culling rates are beneficial in that they:

• shorten generational interval and optimise genetics
• allow for high selection pressure on poor producers
• reduce the average age of the herd and minimise natural attrition
• effectively value add the retained heifers whose sale value at weaning is often lower than the cost of production, and allow higher numbers of old cows to be sold
• increase herd flexibility to change direction and exploit market opportunities.

Table 1 shows the effect of higher retention rate of heifers and the ability to exert more selection pressure on the herd through a higher culling rate of cows over and above culling the empty cows.

Nearly all the negative aspects of poor reproductive efficiency, apart from disease processes, can be attributed to failing to meet the nutritional requirements of breeding herds at particular times in their breeding cycle. Failure in this area is the greatest single cause of economic failure; not failed seasons, poor prices or other things out of our control.

Culling consists of removing unwanted cows from the breeding herd based on:

• poor performance (the same poor performing cow repeats it next year)
• low fertility and late calvers
• aged cows
• other reasons (eg cancer in the eyes, teats, hips and feet).

An example using mathematical modelling

Start with 500 heifers, and assume:

• 88% pregnancy rate to three oestrus cycles
• 2% annual cow mortality
• 3% of cows who calve fail to wean a calf
• no herd replacements.

Table 2 shows the natural rate of attrition before any culling for production takes place. After 8 years, only 112 of the initial 500 cows remain in the breeding herd. This means that, assuming a fertility rate of 88% and a heifer retention rate of 40%, only 12 surplus old cows can be culled to retain a 500-cow herd.

What to measure and when

• Determine the number of heifers to be retained in the breeding herd immediately after pregnancy diagnosis of the heifers.