Often we target for cows in the dry period to obtain a body condition score (BCS) ranging from 3.25 to 3.75. This goal stems from the use of body reserves during the transition period and early lactation to counteract the negative energy balance most lactating cows experience. Body fat is accumulated in a number of body parts in a particular hierarchy. Research in beef cows has shown that this hierarchy of deposition starts with kidney, pelvic, and heart, then the gastrointestinal tract followed by inter- and intra- muscular fat and subcutaneously. In order to support high milk yields in early lactation, body stores of fat are mobilized resulting in the loss of BCS during early lactation that’s typically observed on dairy farms. Ensuring that dry cows reach the optimal BCS allows for a proper transition into lactation. However, as BSC is a visual assessment of fat cover, assessing inter-organ fat provides a total measure of fat stores available to the cow to compensate for possible negative energy balance.
Traditionally researchers and farmers alike have relied on the 5 point BCS system outlined by Ferguson and collaborators in 1994. However, this system relies on trained individuals to be consistent in their scoring approach and assess each cow in a repeatable manner. Moreover, BCS tells us nothing about the internal fat stores. Due to these issues, ultrasound has been used as an alternative to assess total carcass fat. Ultrasound is used on beef cattle and swine are commonly to determine back fat as well as muscle thickness. When the thickness of back fat of dairy cows was assessed in a 2012 trial by van der Drift, back fat thickness decreased by 35 ± 26 % from the week prior to parturition to 8 weeks after parturition. The marked decrease in back fat thickness suggest mobilization of body adipose stores to aid in maintaining energy balance while also showing that not all cows experienced the same onset and extent of mobilization. This difference in back fat mobilization may be due to more than just differences in energy balance; the contribution of other body adipose stores may also play a role.
A newer method to determine total body fat stores utilizes the same principles as van der Drift and others, but takes into account the contribution from both back fat and kidney fat. This method uses a 3.5 mHz transducer that scans the animal at the first lumbar vertebra and the 13th rib for assessment of kidney fat depth and approximately 15 cm for the cow’s midline. Clipping the cow may be necessary to increase image quality and ensure proper measurement. When carcass kidney fat was compared to ultrasound measurements of kidney fat depth there was a strong correlation (r2=0.93), suggesting that assessment of kidney fat depth via ultrasound can provide a reliable means to measure a crucial contributor to whole body adipose stores.