Agri Kultuur December / Desember 2018 - Page 26

14 and 15 months, average DIM were 155, 170, 186 and 201 days and average milk yields were 30.0, 28.9, 27.8 and 26.8 kg/day, respectively. Missing one heat cycle (21 days) amounts to a milk loss of approximately 0.8 kg milk per cow per day. Lactation number The average lactation number of all the cows in the herd provides an indication of the age of a dairy herd. Heifers calve down for the first time at about two years of age after which cows should calve down every year. This means that a cow in fourth lactation is at least six years old. Actual age at the end of fourth lactation can be higher when age at first calving is later than 24 months of age and calving interval is longer than 12 months. The efficiency of production is reduced for cows at higher actual ages while at the same lactation number because of more unproductive days over the cows’ lifetime. The reason why the age (average lactation number) of a dairy herd is important is because the lactation milk yield of cows increases from first to fifth lactation after which it decreases although not declining to the same production level as during first lactation. The efficiency of a dairy herd increases when there are a greater proportion of older cows in the herd. AgriKultuur |AgriCulture First lactation cows The percentage of first lactation cows in a dairy herd provides an indication of the replacement rate in a dairy herd as well as the culling rate in a dairy herd. For an expanding herd, i.e. increasing in size (the number of cows in the herd), the percentage of first lactation cows can be higher especially when sexed semen is being used. For a stable herd, i.e. not increasing in size, the percentage of first lactation cows can be lower although it is greatly affected by culling rate. At high culling rates the proportion of cows in first lactation is higher when aiming to maintain the number of cows in the herd. Increasing the percentage of first lactation cows in a herd would reduce the total (and average) milk yield of the herd because of the lower milk yield of first lactation cows. In closing There are several key performance indicators which can be estimated to monitor herd management. These indicators should be estimated on a monthly basis and the progress (change) should be presented as graphs. This would enable estimating trendlines over time to show progress or deterioration. These trendlines could be used as a basis for changing the standard of management and/or breeding programmes which may include sire selection and cow culling programmes. 26