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Is 150 RFC good enough today?

Jim Paulson, Dairy Extension Educator
January 31, 2015

Thirty years ago, forage testing and the formula for Relative Feed Value (RFV) were being introduced to Minnesota farms. The guideline was 20-30-40. That was based on forage testing 30% ADF (acid detergent fiber) and 40% NDF (neutral detergent fiber) would be 150 RFV with 20% crude protein. This was considered a good goal for the forage quality needed for feeding lactating dairy cows. Thirty years is a long time in terms of research advances in feeding cows and forage analysis. In this time period, we have come to a greater understanding of fiber analysis; incorporating the role lignin plays as an indigestible fiber component. And, we have herds producing 50% more milk per cow per year than 30 years ago. Relative Feed Value was changed to Relative Forage Quality or RFQ. The question in 2015 is: Is 150 RFV high enough for high producing dairy cows today?

Under the RFV system, ADF was used as an indicator of digestibility and NDF was used as an indicator of intake. While NDF still is used as a good indicator of intake, ADF has proven less reliable in predicting digestibility. RFQ uses NDF Digestibility (NDFD) to account for DMI and true TDN, and is calculated (Moore and Undersander, 2002a) as: RFQ = DMI (% of BW) × TDN (% of DM) ÷ 1.23. The TDN in RFQ measures the total energy consumed by the animal, not just fiber, but also CP, NDF, fat, ash and NDFD.

Relative forage quality uses lignin to determine Neutral Detergent Fiber Digestibility (NDFD). This number is a percent of the NDF that is digestible in a given time period in the rumen. Thirty hour NDFD is a common figure used by nutritionists to estimate the fiber digested in the rumen to determine a reasonable energy value for the forage. Thirty hours is fairly representative of the average time forage spends in the rumen. NDF24 and NDF48 are also used as well as uNDF240; which tells us NDF that will not be digested in 240 hours or is considered indigestible NDF. On the forage test below, you can see values for many of these forage fiber measurements. These are numbers that will increasingly come into discussions of your forage by your nutritionist. I have highlighted values you will want to watch more in the future.

What are the RFV and RFQ values for the forage shown below? As I have seen before, the RFV is higher at 145 than the RFQ at 138. It is quite possible to have a lower RFQ than RFV in pure alfalfa hay. It does depend on the lignin content which can vary from cutting to cutting due to weather conditions which may or may not favor greater lignin production. Greater amounts of lignin lower NDFD. Is this forage good enough for high producing dairy cows?

If you look at the forage summaries below, you will see while we are hitting our original average of 40% NDF, a large percentage of the total forage tests at Dairyland Labs are lower than 40% NDF. This would indicate to me that a growing group of alfalfa producers are consciously cutting their alfalfa at lower maturities in order to lower NDF and increase NDFD. This shows up in the second bar graph. If we are trying to achieve NDFD values over 50%, we clearly have to cut earlier. Dairy producers have found that this quality of alfalfa produces more milk and you can feed more of it. It also fits nicely with higher corn silage diets.

On the horizon is lower lignin alfalfa. This alfalfa will be about 10 to 15% lower in lignin. This will improve NDFD and RFQ. This could be a major improvement in forage quality for high producing dairy cows in the future. It might be like a BMR of alfalfa. Our experience with it so far is limited but it will be worth considering. The question will be: Will you continue a 4 cut system and aim for higher quality or consider a 3 cut system for more yield of similar quality that you have now? I believe many dairymen will stay with the 4 cut system. Doing so should enable producers to harvest alfalfa hay/haylage of 175 RFQ. I believe that will be the new number we will be targeting.

Legume Forage Test

Dry Basis % Average Range
Crude Protein %DM 20.33% 19.00 11.58 - 26.42
ADF %DM 31.11% 34.05 23.49 - 44.61
aNDF(w/NA2SO3) %DM 40.18% 44.76 28.16 - 61.36
Lignin (Sulfuric Acid) %DM 7.80% 7.27 4.83 - 9.71
Lignin %NDF 19.56%    
uNDF240 %DM 26.38%    
NDFD 30 (1mm) %NDF 48.38% 41.28 25.10 - 57.46
IVTDMD 30 %DM 65.60% 74.84 65.04 - 84.64
AD-ICP %DM 1.27% 1.46 0.88 - 2.04
ND-ICP (w.o Na2S03) % DM 5.22% 3.26 1.68 - 4.84
Protein Sol. %CP 36.31% 32.90 19.72 - 46.08
Fat (EE) %Dm 2.80% 2.69 1.63 - 3.75
Total Fatty Acid (TFA) %DM 1.41% 2.04 0.98 - 3.10
Ash %DM 13.12% 11.29   8.09 - 14.49

Range of % NDF in Legume samples for 2014.

Range of % NDFD30 for Legume samples in 2014.

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