The use of body lipid as a nutritional buffer is normal in mammalian lactation and some mammals lose up to 50% of their body weight during lactation. This maternal investment in the current offspring must be balanced against long term repeated reproductive success. Animal subsequently replace lost body tissue to achieve that long-term objective. Body lipid reserves are used to sustain early lactation since incremental feed intake can only account for a proportion of incremental milk energy output energy in high yielding cows. The term energy balance is often used to describe the daily body energy state of dairy cows; negative energy balance is associated with body energy loss and positive energy balance with body energy gain. Cows which lose body tissue, and hence energy, in early lactation usually return to positive energy balance at around 40 to 80 days post partum (Coffey et al., 2001; Veerkamp et al., 2000). However, cumulative body energy loss in the first lactation is, on average, only fully recovered at around day 200 (Coffey et al., 2001).  Negative energy balance is related to some health traits (Collard et al., 2000), to resumption of reproductive activity (Veerkamp et al., 2000) and to oocyte size and quality (Kendrick et al., 1999). Individual cows may not regain all lost body energy in the first lactation, leading to a greater deficit to be replenished in the second. The replenishment of body lipid reserves usually occurs later in lactation once pregnancy is established, in preparation for the next lactation. So it forms a part of the complex of competing systems making demands for available nutrients. The outcome of this ‘competition’ creates a cyclical energy balance profile that spans lactations. Parameters of this cycle may be suitable selection criteria for future selection of robust cows since they have a biological interpretation. The phase relates to the period from calving to return to positive energy balance and the amplitude relates to the degree of body energy loss (Coffey et al., in press). The objectives of this work were to 1) calculate energy balance for individual cows over 3 successive lactations and 2) calculate energy balance breeding values for bulls for each day of the first lactation using field data

S. A Barwick

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 2002. Session 10, , 10.2, 2002
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