FROM THE FARM REPORT: STARCH LEVELS IN CALF STARTERS: IMPLICATIONS FOR GROWTH & RUMEN HEALTH

Calf starter plays a critical role in supporting early growth and rumen development in calves, making it an  important component of a successful heifer program. As calves transition to functional ruminants, the rumen needs to develop sufficient physical capacity and absorptive surface area to properly utilize solid feed. Through starter intake, microbial activity and volatile fatty acid production (VFA) are activated, both necessary components in the development of rumen tissues. However, this period also provides many physiological and metabolic challenges for calves. This can lead to a recurring problem that calves face: ruminal acidosis.

During the weaning transition period, ruminal acidosis can occur from highly fermentable diets, which rapidly increase VFA concentration, therefore lowering the pH of the rumen to a point that systemic functions are affected (Gelsinger et al., 2020). The starch level of calf starter has been viewed as the primary factor behind this issue, leading to an ongoing debate on what the ideal starch level is for calves.

Studies over the years have looked at various levels of starch but with contrasting results. One side shows that higher starch levels (23 – 43% of diet DM) increase average daily gain (ADG) and starter intake (Hu et al., 2018). Additionally, higher starch levels are associated with increased propionate production, which plays a key role in stimulating rumen epithelial development (Zitnan et al., 1998). On the other side, lower starch levels have been shown to reduce the risk of acidosis and create a more stable rumen environment (Gelsinger et al., 2020). One way this is accomplished is by incorporating more NDF in the diet, often through forages or a high NDF source in starters. Greater dietary NDF is associated with lower starch content and reduced energy availability for body tissue deposition, resulting in decreased average daily gain (Hill et al., 2008). This creates the challenge of balancing these factors to result in maximizing growth without compromising rumen health.

Dr. Van Amburgh has emphasized the importance of nutrient intake in calves to maximize growth while managing rumen development. Starter intake can be inhibited by high levels of milk and, additionally, by starch content that is too high. He suggests a gradual weaning process that lasts 14 to 21 days to allow enough time for the rumen to expand. Although rumen function and papillae can be developed with lower intakes, complete digestion requires creating a large active microbial population. This is driven by dry matter intake and fermentable carbohydrates, not just starch.

Additional research indicates that the relationship between dietary starch and rumen health extends beyond starch concentration alone. Factors such as intake, ingredient fermentability, and overall diet composition and structure play equally important roles. For example, starch source is critical: Inclusion of barley or wheat in calf starters has been shown to result in a greater decline in rumen pH compared with corn or oats, likely due to the faster fermentability of barley and wheat starch. (Khan et al.,2008). Including forages in the diet can also help increase the pH of the rumen (Suárez et al., 2007). Physically effective fiber stimulates chewing and saliva production, increasing the rumen’s buffering capacity. One study found that providing up to 3% forage NDF maximized intake in calves at 8 to 16 weeks of age, and ADG peaked at around 1.5% forage NDF (Hill et al., 2010). The physical form of starter can also play a role. One study mentioned that ground and pelleted starter resulted in lower rumen pH than textured starter; however, when the calves were provided with chopped straw, those calves consuming ground and pelleted starter had similar rumen pH to those consuming the textured starter (Hu et al., 2018).

Overall calf starter plays a critical role in supporting both growth and rumen development during the weaning transition period. Starch content in calf starter is not a simple one-sided answer, since starch isn’t the only factor playing a role. You have to take into consideration starter form, starch source, and whether forages are being provided. Higher starch can improve growth and rumen development, but it is important to balance that with ingredient selection and forage inclusion to ensure a stable rumen environment.

— Corina Warren

cwarren@whminer.com