MSU Extension

Cows in early to mid-lactation require low-rumen-fill, high-starch diets to achieve their milk yield potential. At this stage, energy consumed is mostly partitioned to milk. However, as lactation progresses and milk yield declines, energy consumed is partitioned to body condition and energy stores mobilized in early lactation are restored. Although adequate body condition is needed for the next lactation, excessive body condition can be mobilized rapidly following calving resulting in depressed feed intake, extended negative energy balance, and increased risk of metabolic disorders.

Recommendations for body condition score (BCS) at calving are typically around 3.25 to 3.5 (1 = thin, 5 = very fat) but some researchers and consultants recommend even lower BCS to enhance feed intake and minimize negative energy balance. Options to manage BCS are limited for cows managed in groups. Use of bovine somatotropin (rbST) partitions energy to milk rather than body condition and can be used on an individual cow basis to increase milk yield and manage body condition. This is no longer a viable option for most Michigan dairy producers and those in some other areas and the only other options are to lower the starch concentration in the diet, which will limit peak milk yield, or to switch cows to a lower starch diet as BCS increases.

Many dairy producers who currently feed one diet to all lactating cows fear that this will result in greatly decreased milk yield. While that was often true in the past when the low-cow management group was fed a diet with sub-optimal ingredients (spoiled silage, feed refusals from other groups, etc.), the goal of a maintenance diet is to maintain BCS (preventing further gain) while also maintaining or increasing milk yield.

Energy partitioning between milk production and body condition varies as physiological state of the cow changes throughout lactation. As lactation proceeds past peak, insulin concentration and sensitivity of tissues increase and energy is increasingly partitioned to body condition, sometimes at the expense of milk yield. While high-starch diets can increase milk yield of high-producing cows, they can result in excessive gain in body condition as milk yield declines and insulin sensitivity of tissues increases.

We showed that a 69% forage diet (0% corn grain) containing brown midrib corn silage increased energy partitioned to milk, decreasing body weight gain while maintaining yield of milk compared with a 40% forage diet (29% corn grain) containing conventional corn silage. In vitro NDF digestibility of the brown midrib corn silage was ~20% higher (55.9 vs 46.5%) than the control corn silage.

In contrast, feed intake and milk yield were reduced when the conventional corn silage was fed in the higher forage diets. We also showed that beet pulp decreased BCS without decreasing yields of milk or milk fat when substituted for high-moisture corn up to 12% of diet DM. Similarly, a recent experiment conducted with cows in the last two months of lactation showed that substitution of beet pulp for barley grain linearly decreased BCS, maintained milk yield and linearly increased ruminal pH and milk fat yield.

Individual cows should be switched to a less fermentable and more filling diet when BCS reaches 3, which will allow a slight increase in BCS (0.25 units) before calving. This will likely increase feed intake and provide a more consistent supply of fuels, partitioning more energy to milk rather than body condition. Furthermore, the less fermentable, more filling diet will increase ruminal pH and decrease risk of milk fat depression.

Forages with a wide range of NDF concentration can be used in these diets but the NDF should highly digestible. More grass can be included in these diets; although grass fiber may have longer retention time in the rumen and be more filling, it is also more digestible. Highly fermentable corn silage should be limited to avoid partitioning energy to body condition. Limit highly fermentable starch sources (e.g. high moisture corn, ground barley, wheat) by substituting less fermentable feeds such as dry ground corn or non-forage fiber sources such as beet pulp. These “flex-fuel” cows (cows placed in the maintenance group) have lower requirements for glucose precursors and can better utilize non-starch feeds to provide energy in a form to spare glucose. Unsaturated fats likely decrease feed intake and increase risk of milk-fat depression and subsequent partitioning of energy to body condition and should be avoided.

Adding a maintenance management group can help achieve the target BCS before dry-off and decrease variation among cows as well as allow lower fill, higher starch diets to be fed at peak lactation allowing them to achieve their genetic potential while preventing over-conditioned cows in late lactation.