We get a lot of calls this time of year about winter cow nutrition and body condition. Cow body condition is a relative term used to describe the level of fatness or fleshiness. Some of the pieces of the puzzle vary with the class (size, age, body condition, stage of reproduction, etc.), but the basics don't vary and the mystery is not very deep. In healthy cattle, it's usually a matter of nutrition.
Each class has specific nutritional needs and uses the total of the nutrients consumed each day in a certain order of priority. The hierarchy illustrated below includes all the tasks we usually ask beef cows to perform throughout the year. It's a simple way to understand and explain many of the problems with nutrition and condition. (Some other hierarchies have the order a little different, but not much.)
For example, take a pregnant cow in a body condition score of 4.5. She needs to gain weight before calving next month. Her hierarchy is in Example 1.
Beginning with "maintenance" at the bottom, she will use her daily intake of nutrients in this order, moving up the hierarchy. Maintenance (MX) includes staying warm, so it can take significant nutrition this time of year. Fetus development (FD) requires slightly more nutrients above MX. If she is to add condition (CD), however, her diet must have excess nutrients above her MX and FD needs.
What are we going to ask of her after she calves? (Example 3)
She's going to maintain herself first. Then, if there are any nutrients left, she'll use them for milk production (MP) up to her genetic potential. MP requires a lot of nutrients. Then, at some point soon, we're going to ask her to breed back, but she likely won't unless she has excess nutrients above MX and MP. If this cow were to add body condition, she'd need excess nutrients above MX, MP and BR. Conditioning is always at the top of any animal's hierarchy. It will take an excellent diet for this cow to gain weight.
It's not exactly that "cut and dried," though. If a cow's daily intake does not supply adequate nutrition for her hierarchy of needs, she will supplement herself with her body stores. Take the previously mentioned lactating cow as an example. If her diet only meets her MX needs, she will mobilize nutrients stored in her body to try to meet her MP requirements. Loss of body fat (condition) is the most noticeable result. The same is true for a dry, pregnant cow whose diet does not meet her MX needs. Even worse, after body fat is depleted, a cow will use other tissues, like muscle and bone, to try to meet her nutritional needs.
If a cow is maintaining body condition, she's meeting all the needs of her hierarchy from her diet. If she's losing (using) body condition, she's not getting what she needs from her diet. The formula is simple: Nutritional requirements - nutrients from daily diet = nutrients needed from supplementation. As managers, it is our responsibility to understand what these needs are and see that they are met at all times for her to perform as expected.
Note: Internal and external parasites get their share of nutrients even before cow maintenance. A sound control program is vital.