Increased corn prices have heightened interest in feeding reduced-starch diets. Results from short-term dairy cattle feeding trials suggest that reduced-starch diets formulated by partially replacing corn grain with high-fiber, low-starch byproduct feedstuffs may be economically feasible.

The objective of this study was to determine the effects of monensin (M) supplementation on lactation performance of dairy cows fed diets of either reduced (RS) or normal (NS) starch concentrations as total mixed rations.

The experimental protocol was approved by the Animal Care and Use Committee of the College of Agriculture and Life Sciences at the University of Wisconsin-Madison. One-hundred twenty-eight cows (90±33 DIM) were stratified by breed (Holstein and Holstein × Jersey crossbred) and parity (primi- and multiparous), and randomly assigned to 16 pens of 8 cows each in a randomized controlled trial in the University of Wisconsin-Madison Emmons-Blaine Arlington freestall, milking parlor dairy facility.

Milk yield was reduced for cows fed the RS diet, but DMI, 3.5% FCM, ECM, and SCM yields, and feed efficiencies were similar for RS compared with NS. Differences in dietary starch concentration or dietary inclusion of monensin had minimal effects on milk FA composition. In this study, RS diets were feasible for reducing corn inclusion in lactating cow diets. Inclusion of monensin at 18g/t of DM improved lactation performance, specifically actual and component-corrected milk feed efficiencies, on both RS and NS diets. We observed few significant interactions of starch and monensin, supporting the use of monensin in diets of both reduced and normal starch concentrations.

Monensin (M) is an ionophore that alters rumen microbial populations and proportion of ruminal VFA. Monensin has been shown to increase ruminal propionate, decrease DMI, and increase milk production efficiency in dairy cows (Ipharraguerre and Clark, 2003b; Duffield et al., 2008). Monensin has also been associated with decreased milk fat percentage and yield (Broderick, 2004; AlZahal et al., 2008) and changes in milk FA proportions (He et al., 2012). These changes are caused by M inhibiting FA biohydrogenation by rumen microorganisms (Fellner et al., 1997) and increased flow of trans-C18:1 FA to the small intestine, which inhibits mammary gland FA synthesis (Bauman and Griinari, 2001). Changes in milk fat composition with M supplementation have been reported to be influenced by dietary factors, such as NDF and NFC concentrations, and delivery method of the diet (Duffield et al., 2003). Previous studies have investigated the effect of M on lactating cows fed diets differing in forage to concentrate ratio (Ramanzin et al., 1997); however, we have found no studies that evaluated the effect of both M supplementation and dietary starch concentrations in dairy cattle. Monensin was not fed in the RS versus NS studies of Gencoglu et al. (2010) and Ferraretto et al. (2011). The objectives of the current study were to evaluate possible interactions of monensin supplementation and dietary starch content on lactation performance and milk FA composition of lactating dairy cows. Our hypothesis was that monensin supplementation would reduce DMI and improve milk production efficiency more with RS than with NS diets. This response could expand the economic opportunities for feeding RS diets to high-producing dairy cows when economically advantageous.

Source: Journal of Dairy Science/M.S. Akins, K.L. Perfield, H.B. Green, S.J. Bertics, R.D. Shaver