Accurate energy prediction in grazed pasture and other forages. Implications of non-protein nitrogen and NDF-D

1. Bill Weiss
Lush Pastures:
Energy and
Protein

2. Do we accurately estimate NE
and MP of diets based on lush
pasture

3. Typical range
Est. DE, MJ or (Mcal)/kg 13-13.8 (3.1 – 3.3)
CP, % 24 – 32
NDF,% 26 – 40
Sugars, % 5 – 18
Fatty acids, % 2 – 4.5
Ash, % 10 – 14
Lush cool season grass pastures
High
CP
FA
Sugars
Ash
Low
NDF

4. 74
3
4
19
12% CP
55
8
10
27
True protein
Inorganic N
Non-AA
organic N
Free AA and
peptides
25% CP
Distribution of Nitrogen in Fresh Ryegrass
Goswami and Willcox, 1969

5. Nutritional Implications: CP
• Potentially high nitrate may be toxicity issue (verify with lab)
• High amount of free AA = high urine N and digestibility
• High RDP = high urine N and high digestibility
• Diet RDP >~11 of DM = 0 MP
• RDP and DMI ? (moderate excess RDP = DMI)

6. Nutritional Implications: CP
 Typical TMR MP/CP = 0.58 to 0.62
 Lush pasture, limited concentrate: MP/CP = 0.23 to 0.40
The only way to increase MP/CP with lush pasture is to
dilute with low CP, high fermentable CHO feeds

7. Amino Acids
(Data from Ian Sawyer and literature)
1. AA-N: ~ 70 to 75% of total N (~25% CP)
• Because of analytical issues, often it is ~50-60% of total N
2. Free AA quite different from protein AA
• Should not assume total AA = RUP AA
• Total AA analysis likely underestimates met. AA supply
• Met = ~1.7 to 1.9% of total CP (~2.3%% of true prot)
• Lys = ~5.1 to 5.5% of total CP (~6.2% of true prot)
• His = ~1.8 to 2% of total CP (~2.3% of true prot)

8. Amino Acids: nutritional implications
1. AA profile of RUP probably not too bad (his, met, lys)
2. Milk protein synthesis driven by ME intake
• Increase ME intake will likely have larger effect on milk
protein than altering AA profile
NASEM Model Estimates
met-His 30% = 2.5% milk protein
met-Met 30% = 3% milk protein
met-Lys 30% = 2% milk protein
DE 5% = 7% milk protein

9. Gross Energy (GE)
Digestible Energy (DE)
Metabolizable Energy (ME)
NE
Fecal Energy
Heat increment
Urine + methane Energy
Retained + Work + Maint
• GE must be estimated
accurately (CP, ash, fat)
• ME or NE must be used
• Equations must include
urinary energy (ME)
• Equations must include
methane (ME)
• Equations should include
adjustment for urea
synthesis (NE)
Pasture Energy

10. 2021 NASEM Energy Supply Scheme

11. Using NASEM to Estimate Energy of Pastures
1. Calculate GE accurately
• Measure ash (high and variable and has 0 GE)
• Use good estimates for fat FA
• Adjust GE from CP fraction
• NASEM uses 5.65 Mcal/kg CP (23.6MJ)
• Lush pasture: high inorganic N and free AA (lower GE)
• Suggest using 5.0 Mcal/kg CP for lush pasture (20.9MJ)
• Work around: Increase ash% 0.16/% unit of CP
Maybe not necessary because of digestibility

12. Using NASEM to Estimate Energy of Pastures
2. Use 48 h IVNDFD rather than lignin
3. FA = 0.57*EE
4. NDF vs NDFom ???
• For most feeds (NDFom = ~0.97*NDF): it doesn’t matter
• Lush pasture NDFom = ~0.80*NDF (?)
• My best guess; use NDF until proven wrong

13. ME = DE
Minus CH4
Minus Urine energy
dNDF, Fat
AA balance, dCP intake, milk protein yield
CH4 = 1.23DMI - 0.145FA + 0.171dNDF
MJ/d kg/d g/kg g/kg (Nielsen et al., 2013)
Urine Energy = Estimate urine N x 14.3 kcal/g N
59.8kJ/g
Estimating Diet ME Values
This will improve NEL estimates for high byproduct diets,
high CP diets and fat supplemented diets

14. NE = ME – Heat Increment
Dietary fiber and FA
Excess RDP/RUP
AA catabolism
NASEM: NEL = 0.66*ME (from Moraes et al., 2015)
This will overestimate NEL for very high protein diets
Need to account for variation in heat increment
or just use ME

15. How does excess CP affect energy values of diets
-271g
-192g
0g
192g
271g
0
50
100
150
200
Change
in
milk
protein
yield
Brun-Lafleur et al., 2010
If Energy is extremely deficient, why does MP milk protein ?

16. How does high CP affect diet energy values ?
Increasing CP:
• Increases GE concentration
• Increases digestibility (increases DE/GE efficiency)
• Increases urine energy (decreases ME/DE efficiency)
• Increases heat production (decreases NE/ME efficiency)
What is the net effect ?

17. Oldham, 1984
Increasing diet CP often increases DMD (DE)
Control diet CP, %
∆
DMD/
∆
%
CP
.04
.02
10 14 18 22
Forage in Ration
Maize Silage
Grass silage
Hay
Avg ~1% unit/ ∆ 1% CP
∆

18. Estimating urinary N on pasture
• Use mass balance (NASEM)
• Total N intake – milk N – fecal N – body growth N
• Method used by NASEM
• Milk N is usually known, body N is minor
• Main source of error would be fecal but error will go in
opposite directions so most would cancel
Recommended Method

19. Estimating urinary N on pasture
• Best equation for confined cattle:
Urinary N, g/d = 0.026*BW*MUN
BW, kg MUN,
mg/dL
Est Urinary
N, g/d
Est Urinary
Energy, MJ/d
500 15 195 11.7 (2.8 Mcal)
500 20 260 15.9 (3.8)
600 15 234 14.2 (3.4)
600 20 312 18.8 (4.5)
• If available, use equation from grazing cows
Option 2

20. 2.0 4.0 6.0
30.0
17.5
22.5
Urine Energy/GEI,%
HI/GEI,
%
USDA
r2 = 0.14
Increased CP increases urine energy and heat increment
20.0
25.0
27.5
Cost is highly variable
• 4 to 8 kcal/g excess N
• Standard is 7.4 kCal (31
kJ)
What is excess ?
• 0.66 ME/NEL had
excess CP
Suggestion:
Excess N = NI – Milk N/0.3
HI = Excess N(g/d)*31

21. Energy if pasture 25% CP (vs 30% CP)
600 kg cow, 20 kg DMI, 30 kg milk, 0.93 kg milk protein
Diet: 23% CP: 88% Pasture (25%CP), 10% corn, 2% mineral
GE, MJ/kg 17.2 17.5 0.98
DE, MJ/kg 13.0 13.3 0.98
Methane: 1.2 1.2 1.00
Urine: 1.2 1.6 0.74
ME, MJ/kg 10.6 10.5 1.01
NEL, MJ/kg 7.0 6.9 1.01
Adj NEL, MJ/kg 6.6 6.4 1.03
-6.5% -8.4%
25% 30% Ratio (25/30)‘
Forage CP 5% units
diet energy 3%
(88% inclusion rate)
Diet

22. Conclusions
1. CP concentrations >~22% has no advantages and
probably reduces diet energy (depending on inclusion)
2. NASEM protein model probably overestimates MP in diets
with high inclusion of lush pastures
3. MP and NEL increase with added starch
• Starch can be a protein supplement

23. Sunrise over the Ohio River