Effect of Replacing Total Mixed Ration with Pasture on Ruminal Fermentation

Authors: VIBART, R - NORTH CAROLINA STATE UNIV; Burns, Joseph; FELLNER, V - NORTH CAROLINA STATE UNIV

Submitted to: Professional Animal Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/14/2010
Publication Date: 7/20/2010
Citation: Vibart, R.E., Burns, J.C., Fellner, V. 2010. Effect of Replacing Total Mixed Ration with Pasture on Ruminal Fermentation. Professional Animal Scientist. 26:435-442

Interpretive Summary: The increasing cost of crude oil is the economics driving the need to use ethanol as a gasoline replacement. The diversion of grain, namely corn, to ethanol production has resulted in increased corn prices, which has been a major staple of dairy rations. Consequently, if dairy products are to remain affordable some shift needs to occur in the composition of the dairy ration that reduces the dependence on corn. This study evaluates increasing use of ryegrass pasture as a replacement of a total mixed ration (TMR) that carries a heavy component of corn (both corn silage and corn grain). In a fall study lactating cows were assigned to 4 treatments consisting of 1) 100% TMR, 2) 79 % TMR/ 21% pasture, 3) 68% TMR/ 32% pasture, and 4) 59% TMR/ 41% pasture. In a repeat the following spring the 4 treatments were 1) 100% TMR, 2) 89% TMR/11% pasture, 3) 79% TMR/21% pasture, and 4) 65% TMR/35% pasture. In the fall, pasture intake increased with the reduction in TMR but milk yields and composition did not differ among treatments. Milk yields (4% fat corrected) averaged 32.3 kg/day for 100% TMR vs. 32.6 kg/day for 59% TMR. In the spring, pasture intake also increased with reduced TMR but milk yields declined, whereas composition was unaffected. Milk yields averaged 32.6 kg/day for the 100% TMR vs. 30.8 kg/day for the 65% TMR. Fall ryegrass remains vegetative and is high in nutritive value, whereas spring growth declines in nutritive value as the plant develops physiologically toward maturation. Because most dairy farms have grasslands which are generally relegated for use by dry stock, there is opportunity to shift some of this grassland into productive forage that could serve as a forage source for the lactating cow thereby reducing the dependence on corn without greatly affecting milk production.

Technical Abstract: Two, 8–week experiments, each with 30 lactating Holstein cows, were conducted to examine performance of animals offered combinations of total mixed ration (TMR) and high–quality pasture. Experiment 1 was initiated in mid October 2004 and Experiment 2 was initiated in late March 2005. Cows were assigned to either a 100% TMR diet (100:00, no access to pasture) or one of the following three formulated partial mixed rations (PMR) targeted at 1) 85% TMR and 15% pasture, 2) 70% TMR and 30% pasture, and 3) 55% TMR and 45% pasture. Based on actual TMR and pasture intake, the dietary TMR and pasture proportions of the 3 PMR in Experiment 1 were 79% TMR and 21% pasture (79:21), 68% TMR and 32% pasture (68:32), and 59% TMR and 41% pasture (59:41), respectively. The corresponding proportions in Experiment 2 were 89% TMR and 11% pasture (89:11), 79% TMR and 21% pasture (79:21), and 65% TMR and 35% pasture (65:35), respectively. Reducing the proportion of TMR in the diets increased pasture consumption of cows on all PMR, but reduced total DMI compared with cows on 100:00. An increase in forage from pasture increased the concentration of conjugated linoleic acids and decreased the concentration of saturated fatty acids in milk. Although milk and milk protein yields from cows grazing spring pastures (Experiment 2) increased with increasing intakes of TMR, a partial mixed ration that was composed of 41% pasture grazed in the fall (Experiment 1) resulted in a similar overall lactation performance with increased feed efficiency compared to an all–TMR ration.