A24-Lancaster Farming, Saturday, June 27, 1998 DHIA Service Center, Orchard Road, University Park, PA 16802 GEORGE CUDOC Pa. DHIA Consultant STATE COLLEGE (Centre Co.) What should you expect from MUN (milk urea nitrogen) values as you pasture your cows? Should rations be changed as we enter the pasture season? These are two important ques tions that need to be addressed to implement pasture usage in our dairy cows’ diets. As always, every farm has unique management applications in feeding dairy cows. What is im portant, is to recognize the driving forces in milk production and how best to implement these on each farm. Optimum milk output is always driven by opti mum rumen function and output. Feeding ru men bugs should always be our number one con sideration whether we cafeteria-style feed our cows, provide a TMR, pasture, or any combin ation of these. At our farm, we feed a 10G percent pure TMR from Dec. 1 through April 1. What happens after April 1 could and would affect our cows if we didn’t address what changes take place. First of all, our main intention is to provide outside, off-concrete exercise April through November. This is best accom plished by our 40 acres of bluegrass pasture for our 160 cows. This pro vides a clean environ ment. ample water, fresh air, and nutritious feed, and a comfortable place to rest four hours a day. These also happen to be the same goals in our free-stall bam, for the other 20 hours. We address the dif ferences in nutrition be tween TMR and grass, on our operation. When we balance our TMR we are mainly concerned with main taining a critical rela tionship between rumen degradable protein and rumen degradable car bohydrates. The correct blend for us has been established using pounds of milk, percent crude protein, percent butterfat, MUN and percent true protein. Averaging, over 80 pounds of milk on twice-per-day milking and maintaining healthy fertile cows presents quite a challenge to not upset the rumen func tion. , Using two measurable compon ents in our diet, such as percent soluble protein and percent starch, we can maintain a similar rumen environment, as we experience changes in our farm feed supplies, or in the need to change feed sour ces for the rations. More specifically, we compare our TMR nutritionally with what replaces it in the four- to five-hour pasture period. To do this, dry matter, (DM) in takes before and after the pasture season starts must be known. Before pasture: DMI=49J pounds DM TMR After start of pasture: DMI=47S pounds DM TMR ?DM Pasture We assume at this point, that the grass DMI is two pounds DM at minimum. From knowing that the better the forage quality, the greater the intake, we actually al low for a 3-pound DMI of pasture. Let’s now compare the two diets: TMR %CP 165% %SP 315% %Starch 30.4% At 49. S pounds intake, 1,169 grams of soluble protein is pro vided and 15.05 pounds of starch, the ratio of grams soluble protein to pounds starch is 77.6. This falls very nicely into the range that on this farm has been established as acceptable using test levels for MUN and other milk components. Now look at what occurs in a pasture situation. Pasture %CP 28% %SP 25% %Starcb 0% When two pounds DM pasture replaces two pounds DM TMR, an additional 1,184 grams soluble protein, and 14.44 pounds starch are provided, calculating out to a ratio of 82. This change is probably not go ing to affect the cow a great deal at this point. During times when we have very high soluble protein levels in our haylage, pasture can be used as a dilution, because fresh forages tend to have a lower percent of soluble protein than en siled feeds. When the need arises, or should I say, when our cows choose to eat more pasture than the above ex ample, we reevaluate and make further changes. For example, at five pounds DM intake of pasture (1,208 grams soluble protein and 13.5 pounds starch) our ratio would be 89.48. We have found this higher ration falls outside of the range of 75 - 85, where our cows function best, and so some adjustments need to be made. What are some of these adjust ments? This is the difficult part because there are many ways to change this ratio back to within 75 to 85, but only a few ways can maintain rumen health. The first obvious change to try is to increase the starch (or carbo hydrate) content of the ration. Generally, in high-producing groups or herds, this is not a good choice because cows are already maxed out. You and your nutri tionist need to look to see if this is a good choice. It would require an increase in starch to 31.9 percent in our TMR to get back to a ration of 85. Another change we may try to substitute TMR ingredients to lower the soluble protein percent age. Again, care should be taken. To obtain an 85 ratio in our cows’ diet we need to lower soluble pro tein to 30 percent This may only be accomplished by substituting with some very high bypass pro tein ingredients that are generally very expensive. The way we have maintained a healthy rumen environment dur ing pasture season has been some what controversial. Crude protein percentage is what we change to maintain a 75 to 85 soluble protein-to-starch ratio. In the example TMR and pas ture we have been following, we lowered the TMR crude protein to 14.5 percent which resulted in a ratio of 80. Milk production did not suffer, MUN values stayed near 11 to 12, and our cows ap peared quite healthy. This has been the third summer in which the diets were success fully adjusted for pasture. (Turn to Page A 27)
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