3 - Implement a female culling and replacement policy to maintain best herd active.

Guidelines for culling heifers and cows

Cull as early as possible, but also at a convenient time, commonly stock is culled at the same time calves are weaned. The initial culling is based on the female being empty at pregnancy diagnosis or having calving difficulties at the previous calving.

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

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

Achieving a balance between the number of older cows in their optimum productive years, for example between their third to sixth gestation, and the influx of new generation heifers as replacements needs to be carefully considered.

What to measure and when

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

Determining culling rate

High culling rates are possible only when the heifer retention rate is well in excess of 60-80%, if aiming to maintain the breeding herd number. High culling rates are also possible if aiming to reduce stocking rates due to poor seasonal conditions or other reasons.

Culling consists of removing unwanted cows from 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).

Mathematical model

To explain how the productivity of a breeding herd declines over time, let’s start with 500 heifers and assume:

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

Table 5: Attrition Rates

Table 5 shows the natural rate of attrition before any culling for production takes place. After 8 years, only 112 of the initial 500 initial heifers remain.

This means that, assuming a fertility rate of 88% and a heifer retention rate of 40%, only around 12 surplus old cows can be culled to maintain the herd of 500 breeding cows.

What to measure and when

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