Fermentable carbohydrates are the main energy source in diets of high-producing dairy cows and play a major role in microbial growth and protein synthesis in the rumen. Neutral detergent fiber (NDF), soluble fiber, starch, and sugars are the main carbohydrate sources. Although these varied carbohydrate sources can be used for energy, they differ in fermentation end-products produced by rumen microorganisms, which in turn alter metabolism and performance by dairy cows.
Starch is rapidly fermented into propionate in the rumen, the propionate is absorbed into the blood, and then it is later transformed into glucose in the liver. Alternatively, starch may be digested in the small intestine directly to glucose. Although starch is not considered a required nutrient for dairy cattle, it directly affects glucose supply; hence, it affects lactation performance of dairy cows.
Corn is the predominant starch source in the United States, but corn prices have drastically increased in recent years following the greater demand of corn for ethanol production. Consequently, this has renewed the interest of dairy nutritionists in formulating lower-starch diets that could reduce corn inclusion without impairing lactation performance.
Replacement of corn grain with non-forage fiber sources (NFFS), highly digestible forages (i.e., corn silage), or sugars are common feeding strategies used to reduce starch in diets of high-producing dairy cows. Typical NFFS alternatives include soybean hulls, beet pulp, citrus pulp, wheat middlings, whole cottonseed, and cottonseed hulls. Potential sugar sources are molasses, whey, deproteinized whey, and sucrose. Viability of these feeding strategies in corn silage based diets during different stages of lactation will be discussed throughout this article.
Fresh cows and early lactation
Early lactation diets should support metabolic adaptations that dairy cows go through during calving. During the initial weeks of lactation, energy intake is unable to meet the energy demand of high-producing dairy cows. Therefore, an increase in dietary energy to minimize the duration of negative energy balance is desired. However, when cows are fed controlled-energy close-up diets, a drastic increase in starch level in a fresh cow diet may have negative effects on rumen health and metabolism.
A trial conducted at Miner Institute evaluated the transition of cows from a controlled-energy close-up diet (15.5 percent starch) to two fresh cow diets fed for 21 days in milk (DIM) that varied in starch content (21 percent vs. 27 percent) by replacing ground corn with a mixture of soybean hulls and wheat middlings. Measured ruminal pH, ruminal lipopolysaccharide, and serum acute phase proteins demonstrated that the risk of sub-acute ruminal acidosis and inflammation increased with greater dietary starch content.
Another trial from Miner Institute showed that lactation performance was better when cows transitioned from a 40-day dry controlled energy diet (13.5 percent starch) to early lactation diets containing either 21 percent starch (fed for 91 DIM) or 23 percent starch (fed for 21 DIM) followed by 26 percent starch (fed for 22 to 91 DIM) compared with 26 percent starch (fed for 91 DIM). The low starch and step-up starch approaches were effective dietary strategies. In contrast, a trial from Cornell University revealed faster rise of intake and milk production when cows were fed a diet containing 26 percent compared with 21 percent starch up to 21 DIM. All cows were fed 26 percent from 22 to 63 DIM. Interestingly, cows were fed a 17.4 percent starch diet pre-partum. Perhaps the difference in starch levels between pre- and post-partum diets may be more important than specific dietary starch levels fed to fresh cows. In addition, it is important to formulate lower starch diets during the early lactation period with digestible carbohydrates so they do not limit intake because of gut fill.
Peak and mid-lactation
Cows reaching peak of lactation also have a high-energy demand, but unless limited by gut fill, those cows should be able to meet their energy requirements by adjusting their feed consumption. For example, in an Ohio State University trial, the substitution of corn grain by conventional corn silage (32 percent vs. 26 percent of dietary starch, respectively) reduced intake and milk, suggesting that the higher corn silage diet increased gut fill and impaired performance of mid-lactation cows. In contrast, a trial from Miner Institute replaced corn grain with brown midrib corn silage (26 percent vs. 21 percent of dietary starch, respectively) and found no negative effects on intake or milk production. The use of digestible forage NDF is important when decreasing dietary starch through increased forage in the diet.
Replacement of starch with sugars or NFFS should not limit intake because of gut fill. Overall, trials that replaced starch with sugars suggested no changes or small increases in lactation performance. Lactation responses to the replacement of corn grain with NFFS (5-10 percent-units decrease in dietary starch), however, have varied effects. Combining the results of short- and long-term trials from Miner Institute and University of Wisconsin, respectively, revealed mixed effects on milk production, intake and feed efficiency when dietary starch levels were lowered to approximately 21 percent. Overall, reducing dietary starch through the inclusion of NFFS may be feasible. However, due to potential negative effects on feed efficiency due to increased intake, monitoring income over feed costs is recommended rather than price per unit of diet dry matter to fully assess economic benefits.
Cows in the final stage of lactation partition energy towards both milk production and body reserves. Thus, formulation of late-lactation cow diets must focus on maintenance of milk production while not allowing cows to gain excessive body condition which could affect the next transition period. Replacement of starch with NFFS or forage NDF during this period is more likely to maintain performance compared with the peak or mid-lactation period. Due to the scarceness of trials during late-lactation, however, caution is recommended when drastically decreasing dietary starch. Although a decrease in starch level during this period is desired, an excessive decrease may result in decreased milk production and forced dry-off, depending on the farm’s specific management practices. A reduction of dietary starch from 27 or 28 percent to 20 or 22 percent will likely have little effect on milk yield by cows in late lactation.
In summary, feeding lactating cows corn silage based diets with low-starch content is feasible. However, supplying an appropriate amount of fermentable carbohydrate is crucial to successfully implement a low-starch feeding program. Furthermore, the use of forages with highly digestible NDF is required to effectively reduce dietary starch through increases in forage content. In addition, replacement of starch with NFFS may alter intake. Thus to fully assess economic benefits, the use of income over feed costs rather than price per unit of diet dry matter is advised.