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Cow selection affects live weight and milk yield

Cow selection strategy

Genetic progress in dairy herds is affected by sire selection. However, despite careful selection procedures, female progeny of selected sires produce at higher, similar or lower production levels than their mothers. This is because of the random combination of genes and the varying effect of environmental factors on the production performance of dairy cows. Dairy farmers should have a system to identify poor performing cows. Such cows should be culled early in their productive lives, i.e. in first lactation, as they will always be at the lower production end of their age group contemporaries. Dairy cattle breeding is generally aimed at increasing the average milk yield of the dairy herd over time, therefore, keeping lower producing dairy cows reduces the average milk yield of the herd. Research has shown that culling lower producing cows during first lactation result in a higher milk yield for the dairy herd.

Recently, modern milking parlours include automatic milk recording systems which in addition often also include weighing cows after milking. This information is used for the daily management of cows. Deviations from the last 10 days for milk yield and live weight (LW) are used to flag cows for management attention. Because of the availability of daily milk yield and LW records for individual cows farmers are using the ratio of milk yield divided by live weight as an efficiency indicator. The value of this ratio has not been really tested although a similar ratio was recommended earlier, in 1940, as 4% fat corrected milk yield (FCM) per lactation divided by the LW of cows at the start of the lactation period. At the time body condition score was not included as a correction factor for LW. Similarly, this is also not the case when using automatic milk recording records. It is presumed that LW is being used as a proxy for feed intake although research in this regard is also lacking. The NRC (2001) use 4% FCM, metabolic live weight (LW0.75), and weeks of lactation in the estimating of dry matter intake. Using this equation shows that the feed intake of cows over the lactation period follows the milk yield curve rather than live weight curve. Both these curves show opposite trends especially in the early part of the lactation period.

Identify poor performing cows

Identifying the production potential of cows depends on their genetic merit and actual performance. Through milk recording, the genetic merit of cows becomes available at birth as the genetic mid-parent value. Recently genomic information is also increasingly being used to identify the production potential of heifers at birth. The popularity of genotyping is increasing because of lower costs while the results are available for an increasing number of traits.

However, the actual production performance of cows is observed during first lactation. Research has shown that first lactation milk yield is a useful indicator of the lifetime performance of individual cows. Furthermore, the milk yield of cows during first lactation is lower than in subsequent lactation periods, and therefore, as a group contributes less to the overall production performance of the herd. Overall feed efficiency of a dairy herd is greater when it has a larger proportion of older cows (higher average lactation number) than a younger herd (lower average lactation number).

Regardless of the breeding programme and the rearing of heifers, as for all populations, a natural variation in milk yield is observed in most dairy herds. The following figure shows the 105-day milk yield for first lactation Jersey cows in a zero-grazing or total mixed ration dairy herd as a ratio to the average of all the cows within the same age group. This means a ratio of 1.00 for a specific cow reflects an average milk yield while cows producing at 20% higher of lower than the average as 1.20 and 0.80, respectively. As expected, the milk yield of cows vary from 0.33 to 1.51 while 70% of cows produce milk between 0.80-1.20 of all cows. Cows below the selection threshold line (0.83) could be culled because of low milk yields.The full article is for subscribed members only. To view the full article please subscribe. It’s FREE!Log In Register

Author

  • Research Associate, Faculty of Animal Sciences, University of Stellenbosch

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