Submitted to: American Forage and Grassland Council Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 7/17/2002
Publication Date: N/A
Technical Abstract: Binding of condensed tannins to protein can prevent excessive proteolysis in forages during ensiling, ruminal digestion, and decay of residues in soil. Plant breeding and biotechnology efforts are underway in the U.S. and abroad to develop alfalfa and other forages with adequate levels of tannin for improving protein and nitrogen use by livestock farms. We used a dairy-forage simulation model (DAFOSYM) to predict the impact of growing and feeding alfalfa with 2% tannin on a farm in southern Wisconsin. Based on limited data from other forages, we assumed that tannins in alfalfa would reduce rumen-degradable protein by 20% and increase acid-detergent insoluble nitrogen by 30%. Tannins were also presumed to reduce the nitrogen mineralization rate of alfalfa residues in soil by 30%. The simulated farm had 100 cows, 85 heifers, and 250 acres of medium silt-loam soil. Alfalfa for silage or hay was grown in rotation with corn grown for silage and grain. Alfalfa was cut with a conventional mower-conditioner or an improved mower-macerator. High-forage rations were fed using homegrown feeds and purchased corn grain, roasted soybean, soybean meal, fat, and minerals. Cows were injected with BST and milked twice daily. Manure was shallow-injected into corn ground in the spring and autumn to minimize ammonia loss. Simulations were run using 25 years of weather data from Madison, Wisconsin. In general, the use of normal or tannin-containing alfalfa did not affect yields of forage or grain. Due to lower nitrogen excretion in manure and nitrogen availability of forage residues, corn-dominant cropping systems required small amounts of additional nitrogen fertilizer when grown in rotation with tannin-containing alfalfa. Milk yields ranged from 27,100 to 28,100 lbs per cow and were greater with rations based on tannin-containing alfalfa or corn silage. Use of tannin-containing alfalfa in place of normal alfalfa reduced protein purchases by 27 to 57 tons, reduced nitrogen losses by 6 to 30 lbs per acre, and increased net return per cow by $62 to $118 per year. Tannins increased the value of alfalfa silage by $24 to $32 and alfalfa hay by $12 per ton of dry matter. Feeding tannin-containing alfalfa shifted grain purchases from lower yielding soybeans to higher yielding corn, reducing the need for off-farm production of potentially erosive and nitrate leaky row crops by 8 to 23 acres. Benefits of tannin were greatest for production systems based on alfalfa silage. Conventional hay systems were not competitive, yielding annual net returns of about $50 less per cow than alfalfa-silage systems and $110 less per cow than macerator-hay systems. We are conducting cropping and feeding studies with birdsfoot trefoil (containing 1 to 4% tannin) and alfalfa to identify optimal forage tannin concentrations and management practices for improving protein and nitrogen use on dairy farms.