Calves typically experience growth slumps after weaning for multiple reasons.

One reason is a change in housing. After weaning, many dairymen introduce their heifers to group housing for the first time. According to a recent USDA survey, most dairy operations house their pre-weaned heifers in individual hutches and after weaning, most heifers are housed in what the survey referred to as “multiple-animal inside area[s].”¹

The switch to group housing brings its challenges as animals must figure out where and how to eat and also sort out their new social hierarchy. This added stress is a contributing factor to the post-weaning growth slump. Stress can also weaken the body’s immune system, causing the animal to become more susceptible to respiratory infections and scours, diarrhea, or other digestive problems during the immediate weeks after weaning. These conditions are the leading causes of weaned heifer mortalities in the US.¹

A second reason calves may experience a post-weaning growth slump is an abrupt change in diet. Prior to weaning, most calves in the U.S. are fed milk or a milk replacer product and a calf starter. After weaning, all bets are off. Some dairy farmers offer high forage diets and minimal grain, others offer a grower feed and supplement with forage, while still others will offer newly weaned heifers a total mixed ration. The decision of what to feed heifers after they are weaned is more often than not a management decision that takes into account existing feed resources on the farm as opposed to a decision based on the heifers and their growing and developing bodies’ needs. This may have consequences for heifer growth and contribute to the so-called post weaning slump.

Compared to research on feeding programs for other classes of animals on a typical dairy farm (pre-weaned calves, dry cows, and transition cows), research for pre-weaned heifers lags behind. However, new work is shedding light on how to feed these animals to better prepare them for the post-weaning period. Results indicate that it may be possible to skip the growth slump with a feeding program that focuses on rumen development.

When a calf is born the rumen is not fully developed and therefore the papillae inside the rumen are not developed. Papillae are the finger-like projections inside the rumen that are responsible for absorbing nutrients. Nutrients are absorbed through the surface area of the papillae, transferred into the bloodstream and used for energy.

“Papillae affect an animal’s efficiency at digesting what it has been fed,” says Kristy Daniels, an animal scientist with Virginia Tech who has been researching papillae growth. “If the calf’s rumen has not been developed properly it will not have optimal nutrient absorption. Therefore the calf will not capture all of the nutrients that are made available through its ration. Nutrients absorbed can be put towards frame growth, lean tissue growth and even internal organ growth. “A well-developed rumen offers the best chance at capturing and making use of fermentation end products,” notes Daniels.

Growth of papillae is directly correlated with what the calf eats, even in the first few weeks of life.²  It is known that the production of volatile fatty acids (VFAs) is essential to the development of the rumen papillae. Acetate, propionate, and butyrate are the main VFAs produced from ruminal fermentation. Research by Heinrichs demonstrated that calves fed milk and high-quality hay diets over a 12-week period had high growth rates, but minimal papillae development compared to calves fed milk only.³  The forage-based diets resulted primarily in production of acetate rather than butyrate which is not optimal for papillae growth. Butyrate or butyric acid is known to stimulate papillae growth in the rumen. To get the levels of butyrate needed for papillae growth, a rapidly fermentable feed, such as starter and/or grain must be fed.

Based on this knowledge, researchers with Purina Animal Nutrition looked at how to optimize rumen papillae growth and have specially formulated a calf starter, AMPLI-Calf^® Grower, to feed from 12 to 24 weeks of age.

Based on controlled research studies at the Purina Animal Nutrition Center during 2010 and 2011, calves fed AMPLI-Calf^® Grower demonstrated 70 pounds more gain (17 percent), 1.2 inches greater growth in height (3 percent) and 4.7 inches greater growth in length (10 percent), 118 percent beneficial growth in rumen papillae length, a 3 percent feed-to-gain ratio and an average daily gain (ADG) advantage of 29 percent at 24 weeks of age.4 During these research trials, all calves received a Full Potential milk replacer diet (28 percent fat, 20 percent protein) until eight weeks of age fed along with AMPLI-Calf^® 22 Starter feed from day 3 to 12 weeks of age. At 12 weeks of age, calves were placed on either 4 or 10 pounds of AMPLI-Calf^® Grower feed per day, with free-choice hay.

An additional study conducted at the Purina Animal Nutrition Center (BT0111; 2011) showed that those calves fed AMPLI-Calf^® Grower feed at 10 pounds per head per day achieved a 62 pound advantage in weight (15 percent), 1.7 inches in height (4 percent) and 3.3 inches in length (6 percent) when compared to calves fed 4 pounds per head per day. ADG on these calves were 0.72 pounds, or 34 percent with an advantage in feed-to-gain ratio of 7 percent.⁴

Five field trials in 2011 (TX, WI (2), MI, & MN) using AMPLI-Calf^® Grower feed showed consistent results of 35 pounds additional weight gain (9 percent) by 24 weeks of age, 1.5 inches in height (3 percent) and 0.31 pounds ADG (16 percent).4

Rumen development also was studied throughout the first two studies (the Purina Animal Nutrition Center; 2011). Quality of rumen development and papillae length were measured on calves from both feeding programs. Those calves fed the higher amount of AMPLI-Calf^® Grower feed demonstrated better rumen wall development (longer papillae length).

“Calves with better developed rumens will result in growthier heifers with no growth slump,” says Daniels. “From there, heifers will be on track to reach other key points in a timely manner like puberty, for instance. Puberty is known to be associated more with body weight than a heifer’s age. Heifers that go their whole life without experiencing a growth slump might hit all of the subsequent “targets” earlier and enter in to the milking herd sooner.”

Work by Dr. Mike Van Amburgh of Cornell University shows that growth rate and nutrient intake prior to weaning has a more direct and significant effect on milk yield than genetic selection for production. Additionally, when a calf is fed a nutrient supply above maintenance, the calf will be set up to become a better lifetime milk producer. His research also showed that a one-pound increase in ADG prior to weaning may increase first lactation milk yield as much as 700 pounds.⁵

“Heifers are not small cows. They are young, growing animals, and their diets should be tailored to them,” says Daniels.

Purina Animal Nutrition recommends a focus on growth rates, nutrient intake and rumen development starting with immediate delivery of high quality colostrum at birth followed by a high quality milk replacer, superior calf starter and AMPLI-Calf^® Grower feed.