Lactation persistency - a trendy management tool Flat lactation curves are believed to be associated with fewer health and reproduction problems. Here you can read more about this trend among dairy farmers from a scientific point of view. TEXT LARS PETER SØRENSEN, GENETIC DEVELOPMENT MANAGER, VIKINGGENETICS Since 2006, the Nordic Cattle Genetic Evaluation (NAV) has calcu - lated breeding values for persistency, which provides information about the shape of the lactation curve. For example, bulls with EBV (Estimated Breeding Values) above 100 (high persistency) will produce daughters that have a flatter lactation curve with lower yield than expected in the first part of the lactation, and a higher yield than expected in the last part of the lactation, compared to bulls with a lower EBV for persistency. Recently, dairy farmers have shown an increased interest in lactation persistency as a management tool because flat lactation curves are believed to be associated with fewer health and repro - duction problems. But how is persistency calculated? Lactation persistency and how to understand it. According to the literature, persistency can be calculated in many ways using the same general definition of persistency. The method used depends on the purpose. For example, if the purpose is to feed the cow as cheaply as pos - sible by replacing concentrates with roughage around peak yield, a measure such as the Y-index at peak may be advantageous. However, such an index is unfavorable if not correlated to lactation yield. According to NAV, breeding values for persistency are expressed as the amount of milk lost or gained due to the shape of the animal’s lactation curve from 100-300 days in milk (DIM). The loss (or gain) is then calculated by multiplying the animal’s EBV at day 100 by 200. This gives the theoretical yield if a constant lacta - tion curve is assumed. This is then deducted from the yield meas - ured by the true genetic lactation curve between 100 and 299 DIM. The larger the derived value is, the more persistent the lactation curve of the animal. Breeding values for persistency are calculated for each lactation and weighted into one breeding value using weights of 0.3, 0.25 and 0.45 for 1 st , 2 nd and 3 rd + lactation, respectively. In the figure below are examples of 3 Holstein bulls, all with breeding values for milk yield equal to 100, but different breeding values for persistency. • V Evertop has EBV = 80 for persistency • VH Evade has EBV = 100 for persistency • VH Leyton has EBV = 120 for persistency. For each bull, a simple lactation average for milk yield is shown as unstandardized EBV expressed as the deviation in kg from an un - specified mean (~6kg). Later peak yield (VH Leyton) results in higher persistency compared to earlier peak yield (V Evertop). Kg milk deviation 1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127 133 139 145 151 157 163 169 175 181 187 193 199 205 211 217 223 229 235 241 247 253 259 265 271 277 283 289 295 301 Table 1. Kg milk deviation and Days in milk 10,000 8,000 6,000 4,000 2,000 0,000 -2,000 -4,000 Days in milk Avg. M Evertop Avg. M Evade Avg. M Leyton Avg. pers Evertop Avg. pers Evade Avg. pers Leyton 2021 | Sire Catalogue 3
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