Extended lactation, the management practice in which the time of first insemination is delayed deliberately, increases calving interval and reduces the proportion of dry cows in the herd. This changes the lactation curve lengthening the descending phase. Therefore, average daily milk production is reduced in cows with extended lactations.
Using the Danish dairy herd model SimHerd, researchers from Aarhus University, Denmark, compared six different extended lactation strategies with the average Danish conventional Holstein farm in which average calving interval was 13 months. The six extended lactation strategies were managing either all, primiparous or multiparous cows for a calving interval of either 15 or 17 months by setting the time of first insemination to 106 or 166 days in milk, respectively.
The authors (Overgård Lehmann et al., 2019) assumed a decline in milk yield between 60 and 305 days in milk of 9.2, 28.7 and 35.0% for first, second parity and older cows, respectively. The SimHerd model dried cows off to ensure a minimum dry period of 49 days or if daily milk yield fell below 15 kg energy-corrected milk (ECM).
The results, published in Livestock Science, showed that while an average Danish herd produced 11054 kg ECM per cow per year, following the strategy of managing all cows for extended lactation at 15 and 17 months reduced cow milk production by 185 (1.7%) and 421 kg ECM/year (3.8%), respectively.
However, as expected, the effects were more significant in multiparous cows. Managing only multiparous cows for extended lactations at 15 and 17 months reduced average production in the herd by 190 (1.7%) and 451 kg ECM/cow/year (4.1%), whereas the reduction in yield when lactations were extended only in primiparous cows was 15 (0.1%) and 40 kg ECM/cow/year (0.4%), respectively,
In conclusion, all extended lactation strategies evaluated in this study reduced cow productivity, therefore, implementation of this management practice in commercial farms is not recommended.
Jesper Overgård Lehmann, Lisbeth Mogensen, Troels Kristensen. 2019. Extended lactations in dairy production: Economic, productivity and climatic impact at herd, farm and sector level. Livestock Science. 220: 100 – 110.
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