Dairy herds consist of lactating and dry (non-lactating) cows, replacement heifers of all ages and often a few dairy bulls. In large commercial dairy herds cows and heifers are mostly artificially inseminated (AI) with semen supplied by semen (or AI) companies. However, because it is sometimes difficult to get some cows (and heifers) pregnant through AI, a few dairy bulls, or herd bulls, are kept as roundup bulls to service such cows and heifers. Many dairy farmers however believe that the best bull is a home-bred bull and for that reason more herd bulls are sometimes being used than what is required as roundup bulls. However, it has been shown that about 75% of herd bulls have a lower genetic merit than AI bulls used during the same time in the herd. The genetic merit of dairy sires being used to get cows pregnant and for the initiation the lactation period affects the genetic merit of cows and heifers in the herd. The genetic merit of cows for milk yield affects the amount of milk cows produce within specific environmental conditions.
The genetic merit of sires is based on the production performance of their daughters. For AI sires this is established in a structured progeny testing programme. This is usually done on a large (national) scale as a minimum number of female progenies must complete at least a first lactation in as many herds as possible. Semen from test bulls in the progeny testing scheme is handed out or sold at a minimum price to dairy farmers to be used on a random basis to service cows in herds. The production performance of all the female progeny of a specific bull is recorded and the bull’s genetic merit estimated from this information. In addition, all production information of cows related to a specific bull is also included in the estimation of his breeding value. A larger number of daughters increase the reliability of estimated breeding values (EBVs) for sires.
For farmers participating in the national milk recording scheme the genetic merit of all the cows, heifers and bulls are determined. This information is provided annually to farmers as the herd’s genetic profile. This makes it possible to determine the genetic merit of a dairy herd as well as genetic trends over years. This can also be compared to the national dairy herd. On receiving this information farmers should evaluate the genetic progress of the herd in consultation with AI company representatives or a consultant knowledgeable about the genetics of dairy cattle breeding. For this article, the genetic information of the bulls used in a Jersey herd in a zero-grazing system is being reviewed.
Dairy sires used
Estimated breeding values for all production parameters for 117 bulls used in this herd were available. One bull was from Australia, six from Canada, 58 from the USA and 52 were recorded as ZAF, of which some (28) with the farm prefix were regarded as herd bulls (ZAF Priv). The rest (n = 24) of ZAF bulls were presumed semen from a local AI company. The mean EBV for milk yield for bulls from Australia, Canada, USA, ZAF AI and ZAF Priv was 512, -112, 268, -4 and -23 kg, respectively. Minimum and maximum EBVs for milk yield for bulls from Canada, the USA, ZAF AI and ZAF herd bulls ranged from -332 to -33 kg, -285 to 1424 kg, -482 to 867 kg, and -953 to 849 kg, respectively. The larger number of bulls originating from the USA and ZAF would have the largest effect on the genetic merit of the herd. The overall mean negative EBV value for the ZAF bulls is a major concern as this would have a negative effect on the milk yield of cows in the herd.
The mean and standard deviation (StDev) genetic merit for different production parameters for all bulls used in the herd are presented in the following table. The large variation in EBV values for the different production traits is reflected in high StDev-values.
In the following figure the mean milk yield EBV for bulls used in the herd show a large variation over birth years. Although the overall genetic trend seems positive, i.e., 9.6 kg/year, the trend was not statistically significant generally because of large variation between years, R2 = 0.053. This means that the overall positive trend was due to change which seems to indicate that no structured genetic programme is being employed to use bulls of higher genetic merit for milk yield over time.
This is a reflection that the breeding objective in the herd does not include a high milk yield and that bulls are selected for other reasons than to improve the herd’s genetic merit for milk yield. This may be that bulls were selected to improve the body conformation traits of the daughters. Body conformation is a poor indicator of the milk yield of dairy cows. Concentrating on improving body conformation traits may be detrimental to overall herd milk yield. Farmers should be aware of the genetic correlation between body conformation traits and milk yield.
The genetic merit of a dairy herd is affected by the sires selected to get cows pregnant. Using herd bulls in dairy herds is a practical way to get cows and heifers pregnant; however, the number of progeny in the herd from such bulls should be kept at a minimum. Such bulls should not affect the breeding objective of the herd which should be maximum profit generated through the sale of milk. Milk sales provide more than 90% of the income of a dairy herd. Sire selection has a long term effect on the genetic merit of a dairy herd affecting its milk yield positively or negatively. Putting too much emphasis on conformation traits could have a negative effect on milk yield. Although the genetic correlation between productive life and body conformation traits for udders and feet and legs is positive, the most important traits for a long productive life is the ability of cows to produce enough milk and to become pregnant soon after calving. Semen cost in a dairy make up about 5% of the total cost of production which means saving on this cost will not have large effect on present profitability, though having a large effect on long term productivity.
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