Rumen Development, Don't Wean Calves Without It!

Successful transition from a milk-based diet to a diet of grain and forages requires proper rumen development. However, rumen development is a process that takes time. Regardless of how much milk you are feeding calves or what age they are at weaning time, if the rumen is not ready, calves are going to struggle after weaning. Failure to prepare the rumen can stall calf growth or put calves at greater risk for illness because of nutritional stress. Both of these situations will cost real money and diminish the value of what you have done with that calf up to the time of weaning.

Rumen development begins when calves start eating solid feeds that enter the rumen. For dairy calves, that first feed is usually starter grain, which contains starch. The bacteria that colonize the rumen come from the environment and from feeds the calf eats. As the calf begins to eat more solid feed, that feed determines the type of bacteria that dominate in the rumen. The bacteria that digest starch produce butyrate and propionate. Bacteria that digest fiber produce mostly acetate. Cells in the rumen wall use butyrate for energy and growth. By giving calves access to starter grain, we encourage the fermentation of starch, which lowers rumen pH and encourages bacteria growth and the production of butyrate. So encouraging grain intake stimulates the growth of rumen papillae, which increases the surface area of the rumen that is available for absorbing the nutrients being produced. Regardless of when the process starts, it takes 2 to 3 weeks for the bacterial population to grow to a number that can efficiently digest grain. This timing is critically important as we try to manage calves through the stress of weaning.

In early weaning systems calves need to begin eating some grain by 2 weeks of age to allow enough rumen development to occur before weaning at 5 or 6 weeks of age. If we do a good job of managing grain intake, it is possible to wean calves at 6 weeks, even when milk feeding rates are high. It is important to remember that deciding to wean calves at a later age or bigger body weight does not remove the rumen development requirement. If calves are drinking a lot of milk, they usually will not eat much grain. Trying to wean calves that have not been eating grain is setting them up for a rough transition.

For an example of a successful weaning transition in calves fed a higher rate of milk replacer, let's look at the results of a study from the Provimi calf research facility (Hill et al., 2012). In this experiment, all calves received milk replacer containing 27% crude protein and 17% fat and were weaned at 42 days. The treatments explored different feeding rates and weaning protocols, which are described in the table below.

Milk feeding rates and calf performance in a study evaluating step-down weaning programs

In this study, all of the calves gained more than 1 pound per day over the 56-day trial, and calves fed either of the HI rate treatments had higher average daily gains than calves fed the LO rate. Calves on the HI rate treatments ate just as much starter before weaning as calves on the LO rate treatment. In the 2 weeks after weaning, the larger calves on the HI rate treatments had higher starter intakes. Calves on the HI rate treatments were fed more than 2 pounds of milk replacer per day and stepped down to weaning over 14 or 21 days. This gradual reduction in milk intake allowed calves time to increase their starter intake before weaning and allowed time for the rumen to adapt before milk was completely removed from the diet.

In a recent Canadian experiment (Steele et al., 2017), calves were fed nearly 3 pounds per day of milk replacer containing 26% crude protein and 16% fat and weaned at 48 days of age. Half of the calves were weaned gradually, with their milk allowance cut by 50% on day 36, and the other half were weaned abruptly. The two groups of calves gained the same amount of weight over the 54-day experiment (approximately 2 pounds per day), but the gradually weaned calves experienced a less dramatic change in average daily gain after weaning (1.83 vs. 0.49 lbs/day from day 49 to 54). Starter intake in the 12 days before weaning averaged 1.17 pounds per day for the gradually weaned group compared to 0.65 pound per day for the abruptly weaned calves. After weaning, starter consumption jumped for both groups of calves, but the gradually weaned calves were better positioned to digest grain and utilize its nutrients, giving them a smoother transition.

When fast-growing, healthy calves on a high rate of milk are weaned, they will often quickly begin eating large amounts of grain. This can be problematic if their rumens are not ready to digest that grain. Due to lower rumen starch digestion, significant amounts of starch may pass through the rumen and eventually end up in the large intestine, allowing bacteria to grow and making calves slightly to moderately diarrheic.

Ease weaning stress by ensuring calves eat at least half a pound of grain per day for 4 weeks or a pound per day for 2 weeks and that they reach 2 pounds per day for 3 consecutive days before weaning. Backing off milk feeding gradually before full weaning can be a helpful strategy for increasing grain intakes, but keep in mind the time required for rumen development (21 days from the time grain is first introduced). Assuming that calves are eating some grain when you start the weaning process, successful step down programs require a minimum of 12 to 14 days to prepare the rumen properly.