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Effects of Swathed Forage Type on Intake and
Total Tract Digestion

 B. W. Neville, G. P. Lardy, T. C. Gilbery, K. K. Sedivec, and J. S. Caton

Department of Animal and Range Science, North Dakota State University, Fargo


 

Highlights

 


 

Table of Contents

Introduction

Materials and Methods

    Animal Diets and Treatments

    Sample Collection

    Laboratory Analysis

    Calculations

Results

Conclusions


Introduction

 

Swath grazing is the practice of grazing windrowed forage. The major advantage to this practice is economic due to the reduction in cost of baling, hauling, and delivering the feed to livestock. While our previous research has investigated cattle performance on swathed forages, in this study we hypothesized that swathed forage type and protein supplementation would also affect dietary intake and digestibility.

 

Materials and Methods

 

Animal Diets and Treatments

All experimental procedures were approved by the North Dakota State University Animal Care and Use Committee prior to the initiation of the study. Five ruminally and duodenally cannulated beef steers (1164 + 70 lb) were used in a 5 x 5 Latin square design to determine intake and digestion of forages from a companion swath grazing study. Treatments were: big bluestem (Andropogon gerardii, BBS), big bluestem with protein supplement (BBS/S), crested wheatgrass (Agropyron cristatum, CWG), foxtail millet (Setaria italica, FM), and native range hay (NR). Steers were offered ad libitum access to the treatment forage over a 17-day period. For the BBS/S treatment, a protein supplement was a cooked molasses block (40 % crude protein, provided at 0.16 % of body weight; Hubbard Feeds Inc., Mankato MN). Steers were housed in an enclosed building in individual stanchions.


Steers were fed at 7:00 a.m. and 7:00 p.m. daily (the protein supplement for the BBS/S treatment was placed on top of the forage) and steers were allowed free access to water and trace mineral salt blocks (Cutler-Magner Co., Duluth MN). There were limited incidences when steers did not consume all of the protein supplement supplied; these times were early in the adaptation period. No such incidences occurred during the collection periods.


All forages were harvested at the Central Grasslands Research Extension Center near Streeter, North Dakota. The forages were collected from a separate swath grazing trial, and for the native range forage, from a site with typical mixed-grass prairie plant species that was selected and harvested at the time of baling.


Sample Collection

The experimental periods were 17 days long, with 10 days of adaptation and 7 days of collection. Feed and ort samples were collected and composited from day 11 to day 17. Animal weights were recorded at the beginning and end of each experimental period. Chromic oxide was dosed intraruminally via gelatin capsules containing 8 g chromic oxide. Dosing began on day 9 and continued through day 17 at 7:00 a.m. and 7:00 p.m. Duodenal fluid samples (0.44 lb) were collected on day 14 through day 16 in a manner that achieved a sampling point every-other hour in a 24-hour day. Samples were composited within steer for each period. Fecal trays were used to collect total fecal matter on days 12 to 17. Fecal output was weighed and sub-sampled (10% wet weight) daily. Samples were composited within steer for each period. Immediately following final fecal collection, samples were mixed with a bowl-mixer and a 10 % sub-sub-sample was dried in a forced-air oven at 50ºC.


On day 13 of each period, ruminal fluid samples were collected at -2, 0, 2, 4, 6, 8, 10, 12, and 24 hours post-feeding. Immediately following -2 hour collection, the rumen was dosed with 200 mL CoEDTA to determine ruminal fluid dilution rate (Uden et al., 1980). A volume of ruminal fluid (200 mL) was collected using a suction strainer, and the pH was recorded using a pH meter and combination electrode. A 3 mL sample of ruminal fluid was added to 1 mL 25% HPO3 in a 12 X 75 mm culture tube. Samples were stored frozen until analyzed for NH3 and volatile fatty acids (VFA).


On day 17, ruminal evacuations were performed at 7:00 p.m. to determine dry matter fill. Rumen contents were removed, weighed, mixed, and sub-sampled for dry matter (DM) analysis. An 8.8 lb sample was taken, and mixed with 2 L 3.7% formaldehyde/ 0.9% NaCl (Zinn and Owens, 1986) for isolation of bacterial cells and analysis of DM, ash, nitrogen (N), and purines.

 

Laboratory Analysis

Feed, ort, and fecal samples were dried in a forced air oven at 50ºC for a minimum of 48 hours. Rumen content and duodenal samples were lyophilized. All samples were then ground in a Wiley mill to pass through a 2 mm screen. Feed, ort, fecal, and duodenal samples were analyzed for DM, ash, N (AOAC 1997), acid detergent fiber (ADF), and neutral detergent fiber (NDF) (Ankom Fairport, NY). Rumen content samples were analyzed for DM (AOAC 1997). Duodenal samples were analyzed for Cr by the spectrophotometer method of Fenton and Fenton (1979). Chromium concentrations were used to calculate digesta flow. Digestibility was calculated by subtracting flow rate from intake, and then dividing by intake. Ruminal fluid was centrifuged 20,000 x g, for 20 min. Liquid was filtered through a 0.45 μm filter, and the supernatant was analyzed for NH3 (Broderick and Kang, 1980). Ruminal VFA concentrations were determined by gas chromaphotography and separated on a capillary column using 2-ethyl butyric acid as the internal standard (Goetsch and Galyean, 1983).


Bacterial cells were isolated from formalized rumen contents. Rumen contents were blended, and the mixture was strained through 4 layers of cheesecloth. Feed particles and protozoa in the ruminal samples were removed through centrifugation at 500 x g for 20 min. The sample was then centrifuged at 30,000 x g for 20 min to collect the bacteria from the supernatant. Isolated bacteria were frozen, lyophilized, and analyzed for DM, ash, N (AOAC 1997), and purines (Zinn and Owens, 1986).


Calculations

Microbial organic matter and N leaving the abomasum were calculated using purines as microbial markers (Zinn and Owens, 1986). Ruminal organic matter (OM) fermented was calculated as OM intake minus the difference between the amount of total OM reaching the duodenum and microbial OM reaching the duodenum. Feed N escape to the small intestine was calculated by subtracting microbial N from total N and thus includes any endogenous and NH3-N contributions. Liquid dilution rate was calculated by regressing the natural logarithm of the Co concentration on sampling time. Ruminal DM, NDF, and ADF disappearance (%/h) of treatment forages were estimated using the model described by Mertens and Loften (1980). The CP kinetic parameters of treatment forages were estimated using the model of Orskov and McDonald (1979) and did not account for potential microbial contamination.


 Results

 

Dry matter intake and total tract digestibility were affected by forage type (Table 1.). Intake of dry matter and organic matter followed similar patterns, and forage type affected intake. Foxtail millet generated the greatest intakes while big bluestem without supplement had the lowest intake level. Supplementation increased dry matter and organic matter intakes of big bluestem by 5.5 and 5.1 lbs., respectively.


Further, when comparing all treatments, foxtail millet had the highest level of total tract digestibility of organic matter: 66.5 % + 4.2. Foxtail millet also had higher total tract digestibility of crude protein, acid detergent fiber, and neutral detergent fiber. Supplementation further increased the total tract crude protein digestibility of big bluestem from 26.5% without supplement to 43.9 % + 6.0 with protein supplementation.


Table 1. Intake and total tract digestion of swathed and standing forages.

Treatmenta

P-valueb

Contrastsb

Itemc

BBS

BBS/S

CWG

FM

NR

SE

Pd

Trt

NR vs. BBS

NR vs. CWG

NR vs. FM

BBS vs. BBS/S

DMI lb/d

11.2

16.7

13.2

20.2

15.0

1.0

NS

<0.01

<0.01

NS

<0.01

<0.01

OMI lb/d

10.1

15.2

12.5

18.7

13.6

0.9

NS

<0.01

<0.01

NS

<0.01

<0.01

CPI lb/d

0.48

1.1

0.9

1.7

0.84

0.1

NS

<0.01

<0.01

NS

<0.01

<0.01

ADFI lb/d

4.6

6.6

6.6

9.5

6.4

0.9

NS

0.03

NS

NS

0.03

NS

NDFI lb/d

8.1

11.4

9.7

14.1

10.6

0.7

NS

<0.01

0.02

NS

<0.01

<0.01

TTOMD

48.6

51.4

48.1

66.5

52.4

4.2

NS

<0.01

NS

NS

<0.01

NS

TTCPD

26.5

43.9

43.1

58.2

40.2

6.0

NS

<0.01

0.03

NS

0.01

0.01

TTADFD

45.5

46.2

43.7

66.9

47.9

5.6

NS

<0.01

NS

NS

<0.01

NS

TTNDFD

48.2

49.2

45.7

66.8

50.6

5.1

NS

<0.01

NS

NS

<0.01

NS

a Treatment abbreviations: BBS = big bluestem, BBS/S = big bluestem with supplement, CWG = crested wheatgrass, FM = foxtail millet, and NR = native range

b Significance: P-value <=0.1, NS is >0.01.

c Abbreviations: DMI = dry matter intake, OMI = organic matter intake, ADFI = acid detergent fiber intake, NDFI = neutral detergent fiber intake, TTOMD = total tract organic matter digestibility, TTCPD = total tract crude protein digestibility, TTADFD = total tract acid detergent fiber digestibility, TTNDFD = total tract neutral detergent fiber digestibility.


Conclusions

 

Of all the forage types consumed by steers, foxtail millet had the highest total tract digestibility and resulted in greater dry matter intake when compared to big bluestem, crested wheatgrass, and native range hay. Furthermore, the addition of a supplement increased dry matter, organic matter, crude protein, and neutral detergent fiber intake, as well as total tract crude protein digestibility, but not total tract digestibility of organic matter, neutral detergent fiber, or acid detergent fiber.


NDSU Central Grasslands Research Extension Center
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