By Rainey Rosemond
Ionophore inclusion in ruminant diets is a widespread practice to increase feed efficiency. In particular, sodium monensin works by exhausting the energy supplies of gram-positive rumen bacteria, eventually causing cellular death. The inclusion of monensin in dairy cattle diets typically increases milk yield per pound of dry matter intake (DMI); however, milk component yield and proportion frequently decrease. A project to analyze the impacts of low-level monensin incorporation into California-style diets was completed on a commercial dairy farm to determine the impacts on DMI, milk and milk component yield, and the mechanisms behind monensin action.
Approximately 500 multiparous, high-producing dairy (106 lbs/cow/d without monensin feeding) cows were used to investigate the impacts of monensin. Cows were fed 11 ppm of monensin that was incorporated in a California-style total mixed ration. Measurements were collected at the end of two five-week experimental periods.
In this study, feeding monensin had no impact on DMI across both sample periods, suggesting that all resulting changes were related to monensin action and not increased dietary energy supply. Monensin had a clear ruminal impact, indicated by increased total tract digestibility of neutral detergent fiber (NDF). Selective action against gram-positive bacteria likely allowed for gram-negative bacteria to proliferate, increasing ruminal fiber digestion. Surprisingly, ruminal outflow of microbial crude protein remained unchanged with monensin feeding. This outcome was likely due to reduced microbial outflow on undigested fiber, which negated the increases in the gram-negative bacterial population. Microbial crude protein contributes largely to the amino acid pool available for milk protein synthesis. Reduced or increased microbial outflow could impact milk protein yield. Increased ruminal fiber digestion likely led to increased ruminal production, the volatile fatty acids (VFA) propionic, acetic, and butyric acids. These particular VFA are absorbed from the rumen and utilized by the animal to create glucose or fatty acids, which contribute to milk component production and yield.
Blood samples were collected to observe mammary inflow of blood metabolites, and results were assumed to show mammary utilization. Blood plasma levels of propionic, acetic, and butyric acids were lower in cows fed monensin when compared to cows not fed monensin. A decline in blood plasma propionic acid was associated with a trend towards increased blood plasma glucose. Similarly, decreased blood plasma acetic and butyric acid concentration is likely because of increased utilization for milk fat production.
Milk yield increased by approximately 8 lbs/cow/day with monensin feeding. Increased milk yield was likely driven by the increased availability of blood plasma glucose, a primary contributor to milk lactose, which is known to drive milk production. Milk fat and protein yields increased proportionally with lactose yields, while milk fat and protein percent remained unchanged.
Milk fat response to monensin incorporation varies widely and is dose-dependent and somewhat dependent on the dietary levels of physically effective fiber in the ration; however, milk fat yield and/or proportion tend to decrease, presumably due to reduced ruminal production of acetate and butyrate (Duffield et al., 2003; Dubuc et al., 2010). In contrast, milk fat yield increased, and percent remained unchanged with monensin feeding in a contemporary California total mixed ration. Increased mammary availability of the VFA fat precursors, acetic and butyric acid, likely contributed to increased milk fat yield. Reduced milk fat yield is assumed to be due to decreased ruminal acetic acid production and utilization; however, an unaltered blood plasma acetic to propionic acid ratio suggests that acetic acid availability changed similarly to propionic. Incorporating monensin in dairy cattle diets should only be done after careful consideration of dietary levels of physically effective fiber levels. If dietary levels of physically effective fiber are low, monensin inclusion could reduce milk fat yield and percentage. The Penn State Particle Size Separator can be used to estimate the physically effective fiber of the diet. Zebeli, et al. (2012) reviewed the importance of physically effective fiber in dairy diets.
In this study, milk protein yield increased proportionally to milk lactose and milk fat yield. As microbial crude protein output remained unchanged with monensin feeding, increased ruminal escape of dietary crude protein to the small intestine likely contributed to increased absorption of essential amino acids and milk protein yields.
Incorporating monensin into California-style diets increased milk production by approximately 8 lbs/cow/day without impacting milk composition or DMI, resulting in a 5.4% increase in feed efficiency, as defined by net energy of lactation density. Increased demand for nutrient-dense foods must be met by increased production efficiency and not by farm expansion. Low-level monensin incorporation in diets of California dairy cows could reduce the number of cows needed to produce the same amount of milk, assuming a 9% increase in milk yield overall, compared to cows not being fed monensin. Incorporating monensin could reduce the feed required to produce a liter of milk.
The impact of monensin inclusion varies and is related to both dose and ration formulation. While there is potential that monensin could improve the milk and component yield on your farm, consulting your nutritionist and examining your feed management program before adding monensin is essential. Having a solid understanding of where your milk and milk components yields are and how they fluctuate historically is necessary before examining the impacts of monensin on your farm. If you choose to add monensin, track your components closely and across multiple seasons as environmental and seasonal diet changes could yield different results.Source : psu.edu