A significant year by grazing level effect was detected on CP content of diet samples collected from 2003 to 2005 at the Gudmundsen Sandhills Laboratory. During drought years (2003 and 2004) cows selected plants which were higher in CP and lower in digestibility. High levels of grazing pressure decreased diet IVOMD compared to diets from ungrazed and moderately grazed pastures. Prediction models generated from these data predict dietary CP and organic matter disappearance (OMD) of cattle grazing native Sandhills range pastures

Adams, Don C.

Geisert, Bobbi Gene

Klopfenstein, Terry J.

Musgrave, Jacqueline A.

Schacht, Walter H.

English

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Nebraska Beef Cattle Reports Animal Science Department 
1-1-2008 
Determination of Diet Protein and Digestibility of Native Sandhills 
Upland Range 
Bobbi Gene Geisert 
University of Nebraska-Lincoln 
Terry J. Klopfenstein 
University of Nebraska-Lincoln, tklopfenstein1@unl.edu 
Don C. Adams 
West Central Research and Extension Center, North Platte, Nebraska, dadams1@unl.edu 
Jacqueline A. Musgrave 
University of Nebraska-Lincoln, jmusgrave1@unl.edu 
Walter H. Schacht 
University of Nebraska-Lincoln, wschacht1@unl.edu 
Follow this and additional works at: https://digitalcommons.unl.edu/animalscinbcr 
 Part of the Animal Sciences Commons 
Geisert, Bobbi Gene; Klopfenstein, Terry J.; Adams, Don C.; Musgrave, Jacqueline A.; and Schacht, Walter 
H., "Determination of Diet Protein and Digestibility of Native Sandhills Upland Range" (2008). Nebraska 
Beef Cattle Reports. 8. 
https://digitalcommons.unl.edu/animalscinbcr/8 
This Article is brought to you for free and open access by the Animal Science Department at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Nebraska Beef Cattle 
Reports by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Page 22 — 2008 Nebraska Beef Report  © The Board of Regents of the University of Nebraska.  All rights reserved.
Determination of Diet Protein and Digestibility of Native 
Sandhills Upland Range 
Bobbi Gene Geisert
Terry J. Klopfenstein
Don C. Adams
Jackie A. Musgrave
Walter H. Schacht1
Summary
A significant year by grazing level 
effect  was detected on CP content of diet 
samples collected from 2003 to 2005 at 
the Gudmundsen Sandhills Labora-
tory. During drought years (2003 and 
2004) cows selected plants which were 
higher in CP and lower in digestibility. 
High levels of grazing pressure de-
creased diet IVOMD compared to diets 
from ungrazed and moderately grazed 
pastures. Prediction models generated 
from these data predict dietary CP and 
organic matter disappearance (OMD) 
of cattle grazing native Sandhills range 
pastures.
Introduction
When grazing native range year-
round, diet quality varies throughout 
the year and with level of grazing 
pressure (1997 Nebraska Beef Report, 
pp. -5; 2001 Nebraska Beef Report, 
pp. 2-25). Lower diet quality during 
dormant months may increase the 
need for protein or energy supple-
ments during these periods to meet 
cow requirements (1997 Nebraska Beef 
Report, pp. -5). Reports of diet diges-
tibilities collected by grazing cattle are 
limited. Lardy et al. (1997 Nebraska 
Beef Report, pp. -5) demonstrated 
that diet DM digestibility of Sand-
hills upland range is the highest in 
June and July and decreased through 
the dormant season. However, these 
digestibility estimates are relative dif-
ferences and in vivo digestibility was 
not estimated. 
Accurate in vivo estimates are nec-
essary to formulate supplements and 
also needed to predict animal perfor-
mance. Geisert et al. (2006 Nebraska 
Beef Report, pp. 109) reported a 5 
percentage unit difference in OMD 
between in vitro and in vivo digest-
ibility of forages. In vivo digestibility 
can be estimated by including a cali-
bration set of samples (with known 
in vivo digestibility) within in vitro 
procedures. Therefore, this study was 
initiated to determine in vivo OMD 
and CP values of Sandhills Range as 
influenced by month, year(moisture) 
and grazing pressure.
Procedure
Diet samples were collected at the 
Gudmundsen Sandhills Laboratory, 
Whitman, Neb., using six esophageally 
fistulated cows. Collections began 
May 200 and continued through 
November 2005. Pastures were chosen 
for sampling based on the stocking 
rate prior to sampling. Pastures were 
separated into three grazing groups: 
non-grazed (None), medium SR (Med, 
0.1 to 0.45 AUM) high stocking rate 
(High, < 0.5 AUM). One pasture was 
not grazed and was sampled at every 
collection time while the remaining 
three pastures varied based on the 
ranch’s grazing rotation. Diet samples 
were collected every  weeks during the 
growing season and monthly during 
the dormant season. Diet samples were 
frozen immediately following collec-
tion, freeze-dried, and ground through 
a Wiley Mill using a 1 mm screen. 
Samples were composited by pasture 
and analyzed for CP and IVOMD.
Precipitation data were collected 
throughout the trial. Moisture at each 
collection time was cumulative begin-
ning October 1 of the previous year 
to two days prior to sampling date. 
Grazing data were recorded to calcu-
late grazing pressure.
In vitro organic matter disappear-
ance (OMD) was based on five forages 
with known in vivo OMD used as 
standards. Three separate in vitro runs 
were conducted and all diet sample 
IVOMD values were adjusted to in 
vivo OMD using regression equations 
generated from the standards. Regres-
sion equations were generated from 
each in vitro run and adjusted for run 
differences using procedures outlined 
by Geisert et al., (2006 Nebraska Beef 
Report, pp. 109). 
Multiple regression analysis was 
conducted to generate prediction 
equations to estimate dietary CP and 
OMD of diets consumed by graz-
ing cattle in the Sandhills. Variables 
included in this analysis included 
moisture, day, and grazing pressure 
(AUM/ton of forage). The day started 
on April 1 of each year and continued 
through March 1 of the following 
year to follow the forage growth pat-
terns. Grazing pressure (GP) was 
calculated as AUM/ton of forage 
produced. Clipped sample data from 
GSL (1998 through 2006) and Barta 
Brothers Ranch (1999 through 2006) 
were used to determine annual forage 
production. Total forage production 
was estimated using the regression 
equation; y = 71.056x + 412.47 (R2 
= 0.575) where y = forage yield and 
x = moisture. The total forage yield 
was adjusted based on forage growth 
curves for the Sandhills region from 
the NRCS using the equation y = 
1.95E07x4 - 1.692 E05x + 0.0498x2 - 
5.244x + 178.284 (R2 = 0.9948) where 
y = forage yield and x = moisture.
Statistical analysis to separate vari-
able differences was conducted using 
the mixed model in SAS. The regres-
sion procedures of SAS were used to 
analyze prediction equations.
Table 1. Year by grazing effect on CP% values 
of diets collected from cows grazing 
upland range pastures.
  Grazing Pressure
Year None2 Med High4 SEM
200 8.5a 8.0a 9.1a 0.6
2004 9.4a 8.6a 8.5ac 0.5
2005 9.5ac 9.0ac 7.1ab 0.4
1Year x grazing pressure interaction (P = 0.04).
2Means un-grazed pastures.
2Means moderately grazed pastures.
4Means heavily grazed pastures.
© The Board of Regents of the University of Nebraska.  All rights reserved. 2008 Nebraska Beef Report  — Page 2 
grazing pressure is applied to a pas-
ture, the availability of highly digest-
ible plants and plant parts decreases, 
forcing cattle to consume diets with 
lower digestibility.
Diet IVOMD was (P < 0.001) 
effected by month (Table 2), with 
diets collected May through July be-
ing more digestible than diets col-
lected during the dormant season. 
Diets collected during the dormant 
season remained relatively constant 
in IVOMD and values gradually 
increased to peak growing season. 
Lardy et al. (1997 Nebraska Beef 
Report, pp. -5) showed similar results 
where digestibility was the greatest 
in the growing season and lowest 
throughout the dormant season.
Regression equations formulated 
from each in vitro run were used to 
adjust the IVOMD values. These 
adjustments allow for comparison 
of samples analyzed in different 
runs. The average adjustment for all 
IVOMD runs for the current trial was 
 percentage units. There was a 2 per-
centage unit difference in digestibility 
comparing the data set from Lardy 
et al. (1997 Nebraska Beef Report, pp. 
-5), who did not adjust to in vivo 
values, to the data generated from this 
trial. When comparing IVOMD data 
from Patterson et al., (2000 Nebraska 
Beef Report, pp 5-6) to IVOMD data 
from this trial, the average difference 
is 5.4 percentage units. This is similar 
to the difference seen by Geisert et al. 
(2006 Nebraska Beef Report, pp. 109). 
Table 2. Monthly average IVOMD and CP (% DM) values of diet samples from native Sandhills upland 
range pastures.
   IVOMD 
Sample Date Ave1 High2 Med None4 CP5
January 54.2 52.2 56.5 5.0 6.9
February 54.6 5.0 55.9 55.0 6.2 
March 52.6 5.5 52.7 52.0 7.4
April  59.5 60.5 60.9 62.8 8.0
May 65.8 62.5 67.5 67.2 12.4
June 62.6 62.6 61.8 6.4 10.8
July 55.9 50.7 57.0 60.0 11.5
August 55.5 50.9 57.9 57.6 8.9
September 51.4 51.6 47. 55.2 8.8
October 5.0 50.1 51.7 56.4 7.9
November 51.4 49.4 50.5 54. 7.6
December 5.9 51.6 51. 58.7 7.0
Average 55.8 49. 55.9 58.0 8.6
1IVOMD average for all pastures.
2IVOMD values for high grazing level.
IVOMD values for moderate grazing levels.
4IVOMD values for un-grazed pastures. 
5CP (% DM) average for all pastures.
(Continued on next page)
Table 3. Organic matter digestibility and CP prediction equations for diets consumed by cattle grazing native Sandhills range pastures
Variable Equation  R2 Model P-value
CP  0.27*Da -4.56E-*D2b +2.86E-5*Dc -8.01E -8*D4d +8.45E-11*D5e +7.88 0.60  <0.001
OMD
 Early Growingf .2825*Mi - 5.759E-4*D2 - 2.0086E-1*M2j - 1.67E- *GP2k +5 4.47846 0.4590  0.0120
 Late Growingg -0.4268*GPl -0.7664*M -0.06015*D +0.01070*GP2 +7.98686 0.71  0.0025
 Dormanth -0.14294*GP -7.77112*M + 0.192*M2 + 0.00271*GP2 + 126.1528 0.5490  <0.001
aMeans day.
bMeans day*day.
cMeans day*day*day.
d Means day*day*day*day.
eMeans day*day*day*day*day.
fMeans growing season beginning April 1 (day 1) through June 15 (day 76). 
gMeans growing season beginning June 16 (day 77) through Sept. 0 (day 18).
hMeans dormant season beginning Oct. 1 (day 184) through March 1 (day 65).
iMeans cumulative moisture.
jMeans cumulative moisture*cumulative moisture.
kMeans grazing pressure*grazing pressure.
lMeans grazing pressure.
Results
Annual precipitation was 1, 15, 
and 19 inches for 200, 2004, and 2005, 
respectively. The average precipitation 
for this area is between 18 and 20 inch-
es annually. A year by grazing pressure 
interaction (P = 0.04) occurred for CP 
of diet samples (Table 1). There was no 
difference in CP among GP in 200 
and 2004 and among Med and un-
grazed pastures in 2005. In 2005 High 
GP decreased CP compared to Med 
and non-grazing. This could be ex-
plained by drought conditions in 200 
and recovering drought conditions in 
2004. Cows may have selected plants 
such as forbs which were generally 
higher in CP, but lower in digestibility 
than grasses. 
There was a year effect (P < 0.001) 
on IVOMD where 200 was higher 
than 2005 with 2004 as intermediate; 
however year did not interact with 
GP. The average IVOMD was 59.1%, 
55.4% and 5.0% for 200, 2004, 
and 2005, respectively. This could 
be explained by decreased precipita-
tion in 200 and 2004 delaying plant 
maturity thus increasing digestibility. 
Grazing pressure significantly 
affected (P < 0.01) IVOMD of diet 
samples where High GP decreased 
digestibility compared to None with 
Med intermediate (54.1%, 55.9%, 
and 58% for High, Med, and None, 
respectively). Grazing cattle naturally 
select plants and plant components 
which are higher in digestibility than 
what is generally available. As more 
Page 24 — 2008 Nebraska Beef Report  © The Board of Regents of the University of Nebraska.  All rights reserved.
However, due to variability among in 
vitro runs, one cannot simply assume 
a constant adjustment percentage. 
The regression equation from samples 
with known digestibility must be gen-
erated for each in vitro run to appro-
priately adjust the data, one equation 
for all runs will not accurately adjust 
each individual run. 
Monthly CP values (Table 2) fol-
lowed a similar pattern to IVOMD 
values (P < 0.001). These patterns 
agree with previous data from Lardy 
et al. (1997 Nebraska Beef Report, pp. 
-5) where CP is highest in the grow-
ing season and lowest during the dor-
mant months. 
Organic matter disappearance 
prediction equations (Table ) were 
separated into three segments; early 
growing (days 1-76), late growing sea-
son (days 77-18) and dormant season 
(days 184-65). Day 1 was April 1 and 
day 65 was March 1 of the follow-
ing year in order to follow the plant 
growing cycle. Significant variables in 
the prediction models varied among 
the three different seasons. Predicted 
OMD values were not different  
(P = 0.9999) from the observed values 
in all seasons. When evaluating the 
prediction of the control pasture (no 
grazing pressure) the model predicted 
similar results as seen in the observed 
OMD results (Figure 1). In 200, low-
er moisture increased diet OMD, and 
increasing moisture in 2004 and 2005 
decreased predicted OMD. In order to 
evaluate the model’s ability to predict 
OMD based on grazing pressure we 
used 2005 moisture data. When graz-
ing pressure was assumed to be high 
(2 Animal Unit Days/ton of forage in 
a deferred grazing system), compared 
(Figure 2) to no grazing pressure, diet 
OMD was lower at any time point 
throughout the year when grazing 
pressure was considered high.
Conclusions and Implications
For producers, nutritionists or 
others to accurately predict cattle 
performance of cattle on pasture 
using the 1996 NRC Model, it is 
Figure 1. Seasonal predicted dietary OMD for the control pasture (none-grazed) during three con-
secutive years. 
80
70
60
50
40
0
20
10
0
O
M
D
, %
200
2004
2005
0 0 60 90 120 150 180 210 240 270 00 0 60
Day
Figure 2. Grazing pressure effect on predicted dietary OMD values. High grazing pressure assumed 
at 32 AUD/ton of forage produced.
70
60
50
40
0
20
10
0
O
M
D
, %
High GP
No GP
0 0 60 90 120 150 180 210 240 270 00 0 60
Day
essential to have appropriate protein 
and energy values for the grazed for-
age. By adjusting in vitro data to in 
vivo, as was done in this experiment, 
we believe accurate energy (OMD) 
values were obtained. By collecting 
diet samples with fistulated cattle, 
the samples reflect what cattle in a 
production setting would eat. Collect-
ing samples over three years differing 
in rainfall allowed us to estimate the 
effect of moisture on diet quality. 
Finally, by collecting samples after 
known amounts of grazing pressure, 
the effect of grazing pressure on diet 
quality was determined.
When all of the data were used in 
the computer model, three complex 
equations were developed for the 
three phases of the growing season. 
The equations account for advancing 
plant maturity (day), moisture and 
grazing pressure. This allows the user 
to predict forage OMD in a variety of 
individulized situtions. Model output 
is illustrated in Figures 1 and 2 where 
one or more of the variables was held 
constant. This model has potential 
for widespread use in Nebraska native 
pastures.
1Bobbi Gene Geisert, graduate student; 
Terry J. Klopfenstein, professor, Animal Science, 
Lincoln. Don C. Adams, director, West Central 
Research and Extension Center, North Platte. 
Jackie A. Musgrave, research technician, Whit-
man. Walter H. Schacht, professor, Agronomy 
and Horticulture, Lincoln. The authors would 
like to express their appreciation to the rest of 
the crew at the Gudmundsen Sandhills Labora-
tory for all of their help and hard work. 


Determination of Diet Protein and Digestibility of Native Sandhills Upland Range

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Nebraska Beef Cattle Reports Animal Science Department 1-1-2008 Determination of Diet Protein and Digestibility of Native Sandhills Upland Range Bobbi Gene Geisert University of Nebraska-Lincoln Terry J. Klopfenstein University of Nebraska-Lincoln, tklopfenstein1@unl.edu Don C. Adams West Central Research and Extension Center, North Platte, Nebraska, dadams1@unl.edu Jacqueline A. Musgrave University of Nebraska-Lincoln, jmusgrave1@unl.edu Walter H. Schacht University of Nebraska-Lincoln, wschacht1@unl.edu Follow this and additional works at: https://digitalcommons.unl.edu/animalscinbcr  Part of the Animal Sciences Commons Geisert, Bobbi Gene; Klopfenstein, Terry J.; Adams, Don C.; Musgrave, Jacqueline A.; and Schacht, Walter H., "Determination of Diet Protein and Digestibility of Native Sandhills Upland Range" (2008). Nebraska Beef Cattle Reports. 8. https://digitalcommons.unl.edu/animalscinbcr/8 This Article is brought to you for free and open access by the Animal Science Department at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Nebraska Beef Cattle Reports by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Page 22 — 2008 Nebraska Beef Report  © The Board of Regents of the University of Nebraska.  All rights reserved.Determination of Diet Protein and Digestibility of Native Sandhills Upland Range Bobbi Gene GeisertTerry J. KlopfensteinDon C. AdamsJackie A. MusgraveWalter H. Schacht1SummaryA significant year by grazing level effect  was detected on CP content of diet samples collected from 2003 to 2005 at the Gudmundsen Sandhills Labora-tory. During drought years (2003 and 2004) cows selected plants which were higher in CP and lower in digestibility. High levels of grazing pressure de-creased diet IVOMD compared to diets from ungrazed and moderately grazed pastures. Prediction models generated from these data predict dietary CP and organic matter disappearance (OMD) of cattle grazing native Sandhills range pastures.IntroductionWhen grazing native range year-round, diet quality varies throughout the year and with level of grazing pressure (1997 Nebraska Beef Report, pp. -5; 2001 Nebraska Beef Report, pp. 2-25). Lower diet quality during dormant months may increase the need for protein or energy supple-ments during these periods to meet cow requirements (1997 Nebraska Beef Report, pp. -5). Reports of diet diges-tibilities collected by grazing cattle are limited. Lardy et al. (1997 Nebraska Beef Report, pp. -5) demonstrated that diet DM digestibility of Sand-hills upland range is the highest in June and July and decreased through the dormant season. However, these digestibility estimates are relative dif-ferences and in vivo digestibility was not estimated. Accurate in vivo estimates are nec-essary to formulate supplements and also needed to predict animal perfor-mance. Geisert et al. (2006 Nebraska Beef Report, pp. 109) reported a 5 percentage unit difference in OMD between in vitro and in vivo digest-ibility of forages. In vivo digestibility can be estimated by including a cali-bration set of samples (with known in vivo digestibility) within in vitro procedures. Therefore, this study was initiated to determine in vivo OMD and CP values of Sandhills Range as influenced by month, year(moisture) and grazing pressure.ProcedureDiet samples were collected at the Gudmundsen Sandhills Laboratory, Whitman, Neb., using six esophageally fistulated cows. Collections began May 200 and continued through November 2005. Pastures were chosen for sampling based on the stocking rate prior to sampling. Pastures were separated into three grazing groups: non-grazed (None), medium SR (Med, 0.1 to 0.45 AUM) high stocking rate (High, < 0.5 AUM). One pasture was not grazed and was sampled at every collection time while the remaining three pastures varied based on the ranch’s grazing rotation. Diet samples were collected every  weeks during the growing season and monthly during the dormant season. Diet samples were frozen immediately following collec-tion, freeze-dried, and ground through a Wiley Mill using a 1 mm screen. Samples were composited by pasture and analyzed for CP and IVOMD.Precipitation data were collected throughout the trial. Moisture at each collection time was cumulative begin-ning October 1 of the previous year to two days prior to sampling date. Grazing data were recorded to calcu-late grazing pressure.In vitro organic matter disappear-ance (OMD) was based on five forages with known in vivo OMD used as standards. Three separate in vitro runs were conducted and all diet sample IVOMD values were adjusted to in vivo OMD using regression equations generated from the standards. Regres-sion equations were generated from each in vitro run and adjusted for run differences using procedures outlined by Geisert et al., (2006 Nebraska Beef Report, pp. 109). Multiple regression analysis was conducted to generate prediction equations to estimate dietary CP and OMD of diets consumed by graz-ing cattle in the Sandhills. Variables included in this analysis included moisture, day, and grazing pressure (AUM/ton of forage). The day started on April 1 of each year and continued through March 1 of the following year to follow the forage growth pat-terns. Grazing pressure (GP) was calculated as AUM/ton of forage produced. Clipped sample data from GSL (1998 through 2006) and Barta Brothers Ranch (1999 through 2006) were used to determine annual forage production. Total forage production was estimated using the regression equation; y = 71.056x + 412.47 (R2 = 0.575) where y = forage yield and x = moisture. The total forage yield was adjusted based on forage growth curves for the Sandhills region from the NRCS using the equation y = 1.95E07x4 - 1.692 E05x + 0.0498x2 - 5.244x + 178.284 (R2 = 0.9948) where y = forage yield and x = moisture.Statistical analysis to separate vari-able differences was conducted using the mixed model in SAS. The regres-sion procedures of SAS were used to analyze prediction equations.Table 1. Year by grazing effect on CP% values of diets collected from cows grazing upland range pastures.  Grazing PressureYear None2 Med High4 SEM200 8.5a 8.0a 9.1a 0.62004 9.4a 8.6a 8.5ac 0.52005 9.5ac 9.0ac 7.1ab 0.41Year x grazing pressure interaction (P = 0.04).2Means un-grazed pastures.2Means moderately grazed pastures.4Means heavily grazed pastures.© The Board of Regents of the University of Nebraska.  All rights reserved. 2008 Nebraska Beef Report  — Page 2 grazing pressure is applied to a pas-ture, the availability of highly digest-ible plants and plant parts decreases, forcing cattle to consume diets with lower digestibility.Diet IVOMD was (P < 0.001) effected by month (Table 2), with diets collected May through July be-ing more digestible than diets col-lected during the dormant season. Diets collected during the dormant season remained relatively constant in IVOMD and values gradually increased to peak growing season. Lardy et al. (1997 Nebraska Beef Report, pp. -5) showed similar results where digestibility was the greatest in the growing season and lowest throughout the dormant season.Regression equations formulated from each in vitro run were used to adjust the IVOMD values. These adjustments allow for comparison of samples analyzed in different runs. The average adjustment for all IVOMD runs for the current trial was  percentage units. There was a 2 per-centage unit difference in digestibility comparing the data set from Lardy et al. (1997 Nebraska Beef Report, pp. -5), who did not adjust to in vivo values, to the data generated from this trial. When comparing IVOMD data from Patterson et al., (2000 Nebraska Beef Report, pp 5-6) to IVOMD data from this trial, the average difference is 5.4 percentage units. This is similar to the difference seen by Geisert et al. (2006 Nebraska Beef Report, pp. 109). Table 2. Monthly average IVOMD and CP (% DM) values of diet samples from native Sandhills upland range pastures.   IVOMD Sample Date Ave1 High2 Med None4 CP5January 54.2 52.2 56.5 5.0 6.9February 54.6 5.0 55.9 55.0 6.2 March 52.6 5.5 52.7 52.0 7.4April  59.5 60.5 60.9 62.8 8.0May 65.8 62.5 67.5 67.2 12.4June 62.6 62.6 61.8 6.4 10.8July 55.9 50.7 57.0 60.0 11.5August 55.5 50.9 57.9 57.6 8.9September 51.4 51.6 47. 55.2 8.8October 5.0 50.1 51.7 56.4 7.9November 51.4 49.4 50.5 54. 7.6December 5.9 51.6 51. 58.7 7.0Average 55.8 49. 55.9 58.0 8.61IVOMD average for all pastures.2IVOMD values for high grazing level.IVOMD values for moderate grazing levels.4IVOMD values for un-grazed pastures. 5CP (% DM) average for all pastures.(Continued on next page)Table 3. Organic matter digestibility and CP prediction equations for diets consumed by cattle grazing native Sandhills range pasturesVariable Equation  R2 Model P-valueCP  0.27*Da -4.56E-*D2b +2.86E-5*Dc -8.01E -8*D4d +8.45E-11*D5e +7.88 0.60  <0.001OMD Early Growingf .2825*Mi - 5.759E-4*D2 - 2.0086E-1*M2j - 1.67E- *GP2k +5 4.47846 0.4590  0.0120 Late Growingg -0.4268*GPl -0.7664*M -0.06015*D +0.01070*GP2 +7.98686 0.71  0.0025 Dormanth -0.14294*GP -7.77112*M + 0.192*M2 + 0.00271*GP2 + 126.1528 0.5490  <0.001aMeans day.bMeans day*day.cMeans day*day*day.d Means day*day*day*day.eMeans day*day*day*day*day.fMeans growing season beginning April 1 (day 1) through June 15 (day 76). gMeans growing season beginning June 16 (day 77) through Sept. 0 (day 18).hMeans dormant season beginning Oct. 1 (day 184) through March 1 (day 65).iMeans cumulative moisture.jMeans cumulative moisture*cumulative moisture.kMeans grazing pressure*grazing pressure.lMeans grazing pressure.ResultsAnnual precipitation was 1, 15, and 19 inches for 200, 2004, and 2005, respectively. The average precipitation for this area is between 18 and 20 inch-es annually. A year by grazing pressure interaction (P = 0.04) occurred for CP of diet samples (Table 1). There was no difference in CP among GP in 200 and 2004 and among Med and un-grazed pastures in 2005. In 2005 High GP decreased CP compared to Med and non-grazing. This could be ex-plained by drought conditions in 200 and recovering drought conditions in 2004. Cows may have selected plants such as forbs which were generally higher in CP, but lower in digestibility than grasses. There was a year effect (P < 0.001) on IVOMD where 200 was higher than 2005 with 2004 as intermediate; however year did not interact with GP. The average IVOMD was 59.1%, 55.4% and 5.0% for 200, 2004, and 2005, respectively. This could be explained by decreased precipita-tion in 200 and 2004 delaying plant maturity thus increasing digestibility. Grazing pressure significantly affected (P < 0.01) IVOMD of diet samples where High GP decreased digestibility compared to None with Med intermediate (54.1%, 55.9%, and 58% for High, Med, and None, respectively). Grazing cattle naturally select plants and plant components which are higher in digestibility than what is generally available. As more Page 24 — 2008 Nebraska Beef Report  © The Board of Regents of the University of Nebraska.  All rights reserved.However, due to variability among in vitro runs, one cannot simply assume a constant adjustment percentage. The regression equation from samples with known digestibility must be gen-erated for each in vitro run to appro-priately adjust the data, one equation for all runs will not accurately adjust each individual run. Monthly CP values (Table 2) fol-lowed a similar pattern to IVOMD values (P < 0.001). These patterns agree with previous data from Lardy et al. (1997 Nebraska Beef Report, pp. -5) where CP is highest in the grow-ing season and lowest during the dor-mant months. Organic matter disappearance prediction equations (Table ) were separated into three segments; early growing (days 1-76), late growing sea-son (days 77-18) and dormant season (days 184-65). Day 1 was April 1 and day 65 was March 1 of the follow-ing year in order to follow the plant growing cycle. Significant variables in the prediction models varied among the three different seasons. Predicted OMD values were not different  (P = 0.9999) from the observed values in all seasons. When evaluating the prediction of the control pasture (no grazing pressure) the model predicted similar results as seen in the observed OMD results (Figure 1). In 200, low-er moisture increased diet OMD, and increasing moisture in 2004 and 2005 decreased predicted OMD. In order to evaluate the model’s ability to predict OMD based on grazing pressure we used 2005 moisture data. When graz-ing pressure was assumed to be high (2 Animal Unit Days/ton of forage in a deferred grazing system), compared (Figure 2) to no grazing pressure, diet OMD was lower at any time point throughout the year when grazing pressure was considered high.Conclusions and ImplicationsFor producers, nutritionists or others to accurately predict cattle performance of cattle on pasture using the 1996 NRC Model, it is Figure 1. Seasonal predicted dietary OMD for the control pasture (none-grazed) during three con-secutive years. 8070605040020100OMD, %200200420050 0 60 90 120 150 180 210 240 270 00 0 60DayFigure 2. Grazing pressure effect on predicted dietary OMD values. High grazing pressure assumed at 32 AUD/ton of forage produced.70605040020100OMD, %High GPNo GP0 0 60 90 120 150 180 210 240 270 00 0 60Dayessential to have appropriate protein and energy values for the grazed for-age. By adjusting in vitro data to in vivo, as was done in this experiment, we believe accurate energy (OMD) values were obtained. By collecting diet samples with fistulated cattle, the samples reflect what cattle in a production setting would eat. Collect-ing samples over three years differing in rainfall allowed us to estimate the effect of moisture on diet quality. Finally, by collecting samples after known amounts of grazing pressure, the effect of grazing pressure on diet quality was determined.When all of the data were used in the computer model, three complex equations were developed for the three phases of the growing season. The equations account for advancing plant maturity (day), moisture and grazing pressure. This allows the user to predict forage OMD in a variety of individulized situtions. Model output is illustrated in Figures 1 and 2 where one or more of the variables was held constant. This model has potential for widespread use in Nebraska native pastures.1Bobbi Gene Geisert, graduate student; Terry J. Klopfenstein, professor, Animal Science, Lincoln. Don C. Adams, director, West Central Research and Extension Center, North Platte. Jackie A. Musgrave, research technician, Whit-man. Walter H. Schacht, professor, Agronomy and Horticulture, Lincoln. The authors would like to express their appreciation to the rest of the crew at the Gudmundsen Sandhills Labora-tory for all of their help and hard work. 

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Determination of Diet Protein and Digestibility of Native Sandhills Upland Range

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