Because the rumen's huge microbial biomass is slow to adapt, change plus heat stress can cause production loss that's hard to recover.
I was recently reminded of how costly major ration changes can be, especially during hot weather. One of my clients ran out of corn silage and was forced to make a major forage change to barley silage.
The barley silage was good quality and well fermented. The ration was adjusted for the change in carbohydrates and protein and the barley was introduced as gradually as possible over about a two-week period. On paper, the nutrient specs of the final ration were very similar. But he still lost about 5 lb. of milk per cow with the switch. Intake took a huge hit during the transition, which unfortunately took place during the first real hot spell of the summer.
Situations like this remind us that we are not just feeding a cow; we are feeding a huge microbial biomass in the rumen. This microbial community is slow to adapt. Some members of this bug community die, others adjust to the change, and a new biomass eventually thrives.
This happens anytime a ration change takes place, but it can be a major issue with large forage changes coupled with environmental stress. Production lost during the summer is often hard to recover. This is especially true with post-peak cows. During heat stress, nutrients are being redirected away from the mammary gland for maintenance of other tissues. Any interruption in the flow of the rumen microbial biomass caused by a ration change further reduces the nutrients available for milk production.
Plan your forage needs to avoid major changes during the summer months. Try to balance your rations before heat stress kicks in and stick with the same ration during the summer. We are feeding a microbial population that doesn’t like change. Consistency in feeding is more important than a precisely balanced ration.
Intake and production are slowly climbing on my client’s dairy as the rumen adjusts to the new forage. But I doubt that we will get all the milk back until fall when the herd turns over with new calvings.