Source: Michigan State University Extension, Jerad Jabore
Proper management of young calves is essential for their success throughout life. It is critical to ensure that each calf receives an adequate amount of high-quality colostrum for the transfer of immunoglobulins from the dam to the calf to support the immune system of the calf. Next, we must also consider the importance of rumen development, the energy and protein nutrient requirements needed for the desired growth rate, and the rumen environment needed to maintain proper gut health.
One of the greater expenses of raising young calves is the feeding milk replacer as opposed to a dry feed ingredient-based diet. To wean the calf from milk and make the switch to a dry feed ingredient-based diet, the rumen must undergo further development to ferment the dry feed being consumed so the nutrients from the feed can be absorbed by the calf. Therefore, it is economically advantageous to encourage rumen development of calves at a young age.
Rumen development is stimulated by the volatile fatty acids (VFA) produced from rumen fermentation of feed ingredients rather than the physical characteristics of forage (i.e., stretch and scratch factor). Therefore, both grain and forages can stimulate rumen development, while milk alone does a poor job. However, grain-based diets produce a greater concentration of propionate and butyrate, the two VFA most important for papillae growth in the rumen. Comparatively, forage-based diets produce a greater concentration of acetate. Grain-based diets are more digestible and energy dense than forage-based diets which allow them to elicit the greatest rate of growth by the animal. While forages are less energy dense and can limit feed and energy intake due to gut fill because of the bulkiness of long-stemmed forages.
As a result, it would make sense to supplement young calves with a grain-based diet (i.e., starter) until they are able to make a complete switch after weaning to promote the greatest rate of rumen development and body weight gain. As such, calf starters normally contain a large concentration of rapidly fermentable carbohydrates, the digestion of which produces VFA, which can overwhelm the rumen’s ability to maintain a healthy pH for proper function. The physical form of the starter also affects the rate of fermentability within the rumen, with finely ground and pelleted starters being more rapidly fermentable compared with textured starters.
After consuming a meal, rumen pH decreases, becoming more acidic, before returning to normal before the next meal is consumed. Acidosis occurs if the rumen pH fails to return to normal or remains low (<5.6) for an extended period of time, causing poor rumen function resulting in symptoms of anorexia, lethargy, and diarrhea. On the other hand, forages require more chewing and increase the production of saliva that contains bicarbonate, which can buffer (i.e., increase) the rumen pH. Forages are abrasive and can prevent keratinization of the rumen epithelium that hinders VFA absorption in the rumen. For pre-weaned calves with no access to consume bedding, a study in The Professional Animal Scientist observed voluntary straw intake was approximately 4 to 5% of dry matter feed intake, while weaned calves voluntarily consumed straw at approximately 1% dry matter feed intake. Voluntary straw intake demonstrates a need for fiber intake to maintain rumen health and function of young calves.
An article in Applied Animal Science reported that forage inclusion in young calf diets is complex and depends on the physical form of the starter, grain source in starter, starch and fiber concentration of the starter, forage source, forage particle size, forage amount, and bedding source. The forage requirement for pre-weaned calves may be extremely low due to a lesser starter intake while milk replacer represents a larger proportion of their diet. As starter intake increases, so does the fermentation of carbohydrates supplied by the starter that can increase ruminal acid load and the potential for acidosis. Forage inclusion at approximately 5% of dry matter feed intake can result in similar average daily gains and feed efficiency compared with 100% grain-based diets, likely due to the improved rumen environment and health. Forage inclusion greater than 10% or with long-stemmed forages have the potential to add too much bulk and gut-fill that restricts starter and energy intake, thus limiting average daily gain and feed efficiency. Starters that are highly processed and fine, resulting in rapid fermentation and VFA production, have shown a greater beneficial response to forage inclusion compared with textured starters that have a slower rate of fermentation in the rumen. An article in The Professional Animal Scientist reported regarding forage source, average daily gain and dry matter feed intake are maximized when the neutral detergent fiber (NDF) provided by the forage is between 0 and 2%. Therefore, more alfalfa hay could be included relative to grass hay, while more grass hay could be included relative to straw because of differences in NDF% of each forage.
In conclusion, forage inclusion in the diets of young calves can help maintain rumen pH for proper rumen health and support rumen development. Low inclusion rates of forage (<5% of dry matter) can provide the beneficial effects of forage NDF needed to maintain rumen health and maximize calf growth. Clean straw bedding that is applied frequently and in slight excess may satisfy the forage needs of young calves consuming grain-based diets. If you have follow-up questions, please contact the Michigan State University Extension Beef and Dairy team educators for additional information.