Closing your herd and using home bred bulls can prove an effective and economic approach to improving herd performance. Selecting top breeding sires for use in the herd is based on exactly the same principles as buying bulls from a stud breeder. Your herd must be large enough to provide the genetic variability and permit the culling and selection pressure necessary to make good genetic progress.
Defining the safe minimum size of a closed herd depends on circumstances but as a reasonable guideline the herd should not have fewer than 150 breeding cows and five bulls over a prolonged period.
Genetic variation/inbreeding
To ensure high genetic variability before you close the herd buy in bulls from several different herds for at least the two previous breeding seasons. Using AI from a range of sires achieves the same object. This also overcomes the potential problem of inbreeding. If bulls from just a few small herds (less than five) have been used in producing the last two or three generations of cows, the herd is probably already partly inbred.
Once the herd is closed the rate of inbreeding is mainly determined by the number of new bulls used each year, and the number of years for which they are retained. Fig. 1 shows the rise in the proportion of inbreeding in different-sized herds. To minimise inbreeding use bulls for only one or two seasons. This of course is also consistent with maintaining high genetic selection pressure. Using bulls for two years is cheaper than using them for one. It increases the generation interval by three months, but also increases selection pressure by 16% — 20% (because fewer, higher index bulls can be used). When bulls are used for two years the rate of inbreeding is 1.5 — 2 times greater than when they are used for just one year because more calves are sired by each bull. To reduce the inbreeding levels it is probably wise to buy the occasional bull from an unrelated herd or use AI.
Figure 1 The amount of inbreeding which results from different herd sizes and bull disposal policies.
