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The effect of longevity of the herd is of interest because of erosion and therefore a high culling rate. This is the usual topic of discussion. What is less discussed is the general reduction in the lifetime of cows because of the steady increase in milk yield brought about by improved management, feeding, housing and genetic merit. Another factor affecting longevity is economic pressure, resulting in increasing herd sizes to improve returns on investment capital. The effect of increasing herd size on longevity is less easy to define, but the speed of herd growth may occur without a concomitant improvement in management and housing conditions, resulting in deteriorating cow environmental and social conditions, and as a result reduced milk yield. This may lead to an increase in culling rate. This is problematic as such culled cows are a loss because they leave the herd before reaching their lifetime production potential. Also, the cost of milk production increases because more heifers have to be reared to replace these cows. The question however is whether this issue is that simple.
An indicator of the average age of a dairy herd is the average lactation number of all the cows in the herd. A herd with a low average lactation number is usually an indication of a high culling rate and it is also a reflection of a high percentage of first lactation cows in the herd. The opposite scenario provides the opportunity for internal herd growth or in the case of a stable (not expanding) herd for surplus heifers to be sold for additional income. Also, as an alternative, a portion of the herd could be inseminated with beef sire semen to increase the meat output of the herd. An approximation is that a stable herd (not growing in numbers) would have about 25% first lactation cows, whereas in an expanding herd the number of first lactation cows would be about 33%. The number of older cows (5th lactation and older) in a stable herd would be 28% and in an expanding herd 18%. Calculated on this basis, the average lactation number for an expanding and stable herd would be 2.8 and 3.4 respectively. Presently from milk recording data, the number of first lactation cows in herds varies from 20 to 40%, depending on culling rate.
According to the authors cited below, there is a general belief among Western Cape dairy farmers that a younger herd, meaning a lower average number of lactations per cows with fewer older cows, is easier to manage than an older herd. This is opposite to the general belief that cows should get the best opportunity for a long life to unlock their potential production in later lactations. The effect of herd structure in terms of the spread of the number of cows per lactation on the total milk yield of a dairy herd is therefore of interest. Consequently, the aim of the authors was to study the effect of herd structure on the production performance of Holstein and Jerseys herds.
In the study, production records of Holstein and Jersey cows were obtained from the ARC Milk Recording data base for all milk recording years from 2003 to 2012. Records were sorted for lactation number from low to high and mean milk, fat and protein yield, and fat, protein and lactose percentages per lactation were estimated. Herd structures were constructed for herds with a 20, 30 and 40% replacement rate as indicated by the percentage of first lactation cows. Herds were standardized to 1000 cows for each herd structure option. The effect of herd structure on the total herd milk yield was estimated by multiplying the milk yield of Holstein and Jersey cows with the relevant number of cows per lactation category. A calving interval of 365 days was used for all herds. All the lactation category milk yields were summed for each herd.
The results, as expected, showed a high erosion rate for both breeds with the number of records in lactation 5 being only about 7 and 9% of the total number of records for Holsteins and Jerseys respectively. The number of first lactation records made up 29 and 26% of all Holstein and Jersey records respectively. Whereas the number of first lactation cows surviving to the 5th lactation was 34% for Jersey herds, this figure was only 23% for Holstein herds. The total number of records from lactation 5 to lactation 10 was about 13 and 21% of all Holstein and Jersey records respectively.
For both breeds, average milk yield per lactation increased from the first to the third lactation. Whereas Holsteins showed a downward trend from the fourth lactation and recorded milk yields lower than first lactation cows at the 7th lactation, Jersey cows peaked at the 4th lactation after which a very slight downward trend was observed. The milk yield of Jersey cows in the 10th lactation was 6.6% higher than the first lactation cows whereas for Holsteins the milk yield was 14% less than that of the first lactation cows. The authors attributed the difference between the two breeds to a higher annual increase in genetic merit for Holstein cows in comparison to Jersey cows, resulting in a faster annual increase in first lactation milk yield. Another reason could be that Holstein cows might also have been culled earlier than Jerseys because of low milk yields or other management problems such as recurring mastitis and failure to conceive. Therefore, Holstein cows that did manage to survive might have been lower producing cows not affected by factors such as mastitis and reproductive failure.
In terms of the effect of replacement rate on herd structure the following was evident: At a 40% replacement rate first and second lactation cows comprised 69% of all cows in the herd whereas for a 20% replacement rate the number was 38%. With an increase in the number of first lactation cows, herds of fixed sizes were made up by fewer older cows (cows in 5th to 10th lactation). At a 40% replacement rate, cows in these age group categories constituted only 7% of all cows whereas in a herd with a 20% replacement rate it was 32%. The average number of lactations in herds with replacement rates of 20, 30 and 40% was 3.61, 2.86 and 2.16 respectively. The reduction in the average number of lactations is the result of more first and second lactation cows relative to all the cows in the herd. The estimated total milk yield of herds with different replacement rates varied little within breed although yields were affected differently for the Holstein and Jersey herds. In the Holstein herds, if the number of cows in the 7th to the 10th lactation increased, herd milk yield was reduced whereas in Jersey herds this was not the case.
The effect of replacement rate on total milk yield is interesting. Compared with a herd with a replacement rate of 20%, the total milk yield of the Jersey herds was 1.8 and 3.7% lower at replacement rates of 30 and 40% respectively. At higher replacement rates the number of older cows was reduced resulting in a decrease in the total herd milk yield. For the Holstein herds using the same replacement rates, total herd milk yield was approximately maintained at the same level. The reason is because the higher milk yield levels of older Holstein cows were not maintained beyond the 6th lactation.
In conclusion, these results suggest that it would not have been advantageous to keep the Holstein cows beyond the 7th lactation as was the case for the Jersey cows. However, the results of this study should be interpreted cautiously as the milk yield and herd structure of individual herds may vary from the examples used, since milk recording data represents but a small percentage of national herds. What is necessary in future studies is that the effect of rearing cost of large numbers of replacement heifers in herds with high replacement rates should also be included in a total herd cost-analysis. For herds requiring a low replacement rate while being kept stable, alternative income streams could be developed such as selling surplus heifers as breeding animals or using a beef sire on below average genetic merit cows for marketing progeny as veal or beef.