Of course, not all cows on the farm produce the same amount of milk and production of milk per cow varies from farm to farm.
Knowing the variation of milk within the farm or animal groups can be very useful in managing the herd.
Grouping cows is done to reduce the variation within the group and to provide for more focused management toward the needs of the respective group.
However, variation in milk yield within a given farm can be rather large, and the range of variation among groups within a farm can be quite large (e.g., 12 to 35 lb standard deviation; SD). Calculating the mean, median, and SD of milk for the farm and each group can be very useful to herd management in the following manner:
Nutrition. Formulation of diets for dairy cows necessitates the adjustment of the milk yield above the average for a herd/group so that the higher producing cows are less compromised. Having the SD available can be very helpful to the nutritionist in formulating diets. A practice used by some nutritionists is to balance a diet for a respective group at one SD above the mean.
For example, if the average milk yield is 70 lb and the SD 15 lb for a group, then the diet may be balanced for 85 lb of milk. Variation within farms may be higher than one expects, resulting in excessive underfeeding of the higher producing animals in the group.
Grouping strategies. Large variation within groups may reveal that the grouping strategy is not meeting the target for the how the groups are defined or simply just should be changed. Also, the mean and SD of milk for groups can help identify which groups can basically receive the same ration.
For example, in a recent discussion with a dairy farmer, this information was used to encourage the farmer to move from a single TMR fed to six different groups to two different TMR being fed to the herd. These types of decisions can have major impacts on income over feed costs.
Culling. When you begin to look at groups with high SD, then you can identify cows that are really low yielding compared to the mean of the group. Then the questions that must be asked are “Is this cow in the wrong group?” or “Does this cow need to be culled?”
Looking at records from several farms, it is apparent that there are often several “loafers” in groups that either need to go to market or to another group.
These data from within a farm are directly available from some dairy management software, and for others, the milk yield can be imported into a spreadsheet where the mean and SD can be calculated.
Making use of such data on variation in milk yield in formulating rations and managing cows can improve nutrient availability to animals and improve income over feed costs and thus farm profitability.
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