Response of Lactating Holstein Cows to Diets Varying In Fiber Source and Ruminal Starch Degradabllity

¹ Department of Animal Sciences, University of Arizona, Tucson 85721

Twenty-two Holstein cows near peak lactation were used in an 8-wk experiment to study the effects of forage fiber source and ruminal starch degradability on lactational performance. High and low digestibility forages (alfalfa hay and wheat straw) and sorghum grain processed to give low and high ruminal starch degradability (dry-rolled and steam-flaked) were factorially arranged in four treatments. Diets contained 30% total NDF and 22% forage NDF; consequently, forage to concentrate ratio decreased from 52:48 to 31:69 when straw was substituted for alfalfa hay. Source of forage fiber did not influence production characteristics. Ruminal passage rates for liquid and diet ingredients were slower when diets contained wheat straw, but chewing time and digestibility of NDF were not affected. Total tract digestibility of starch increased from 80 to 97% when grain was steam-flaked, but digestibility of NDF was not influenced. Diets containing more ruminally degradable starch resulted in higher yields of milk (+3.4 kg/d) and milk protein (+110 g/d), milk protein percentage (+.08%), and improved efficiency and persistence of milk yield. Milk fat percentage decreased from 3.6 to 3.2% when steam-flaked grain was fed, but daily yield of milk fat was not decreased. Increasing ruminal degradability of starch has the potential to improve lactational performance when the proper balance of ruminally degradable starch and forage fiber is maintained.

Key Words: fiber • starch degradability • lactating cows

Submitted on October 19, 1992
Accepted on February 24, 1993

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