1. Anatomy of the Ruminant Stomach
The ruminant stomach has developed into an organ that provides extensive pre-gastric microbial fermentation. While microbial fermentation takes place in the gut, primarily the colon and cecum of all animals, pre-gastric fermentation is restricted to ruminants (and pseudo-ruminants such as llama, alpaca, and the camel). The stomach of the ruminant is divided into 4 compartments: reticulum, rumen, omasum, and the abomasum.
Extensive pre-gastric microbial fermentation
4 compartments

3. - The stomach is very large in proportion to body size and occupies nearly 3/4 of the abdominal cavity.
- The reticulum and the rumen are joined
- The rumen is divided by folds into several sacs: the cranial sac, the dorsal sac, ventral sac, the dorsal blind sac, and the ventral blind sac. The rumen cannulae is usually placed in the dorsal sac.

4. Cow rumen

5. Cow reticulum

6. The interior surface of the rumen
Reticular epithelium
The inside of the omasum

7. Lining or epethelia of the reticulo-rumen cattle (left), sheep (right)
The lining or epithelium of the reticulo-rumen is unique. The reticulum is characterized by a honey-combed shaped surface. The papilae are most evident in the cranial and ventral sacs where more of the absorption occurs.
cattle (left), sheep (right)

8. Papillae most dense in the ventral sac where most absorption occurs
The papillae are darkest and densest in the ventral sacs, and these are the papillae that are most often damaged (or keaitinized) first in the case of acidosis. One way of estimating this kind of damage is to biopsy the ventral papillae.

9. Papillae development Fermentation/absorption driving factor
This slide shows the development of papillae from the newborn stage, through the milk stage to a fully-functioning rumen. The volatile acids produced during digestion are thought to be the driving factor of rumen development.
Fermentation/absorption driving factor

10. This is the right view of the stomach - you can see the omasum, abomasum, and the starch of the duodenum which are located to the right of the reticulo-rumen. That is why we cannulated the small intestine from the right side of the animal.

13. Reticular groove (pinned in an open position)
cardia
Ventral groove
- The cardia is the terminal end of the esophagus in the rumen. This is where digesta enters the reticulo-rumen.
- Below the cardia is the reticulo-omasal orifice. This is where digesta passes out of the reticulo-rumen.
reticulo-omasal orifice

14. Reticular groove (in a closed position)
esophagus
The reticular groove can close, effectively creating a direct channel from the cardia to the reticular-omasal orifice. With young calves fed milk (or milk replacer) the sucking reflex stimulates the groove to close such that the milk bypasses the rumen, passing directly to the omasum and then the abomasum. Closure of the groove does not occur for cattle fed solids.
reticular
groove

15. Omasum tightly packed with digesta (left) leaves or folds
This is a cross-section of the omasum. On the left side, the omasum is tightly packed between the numerous folds with digesta that is trapped. Washing out the digesta gives a better perspective on the leaves. The leaves are thought to trap the larger particles that leave the rumen.
tightly packed with digesta (left)
leaves or folds

16. Cow omasum

17. Function of the Omasum
acts as a filter to sort out liquid and particles for passage to the abomasum
selective retention of the coarse particles
absortive site for water, VFA, Na, K, etc
reduction of the net volume entering the abomasum

18. Abomasum fundic folds (unique) duodenum fundic pylorus pyloric region
This is a cross-sectional view of the abomasum with the fundic and pyloric regions. The digesta leaves the abomasum to the duodenum via the pyloric sphincter.
fundic
fundic folds (unique)

19. Cow abomasum

20. Function of the Abomasum
digesta to abomasum continuous
thus, continuous secretion of gastric juice (pepsin. HCl)
stimulated by VFA and lactic acid
short retention time (< 3 h)
digesta entering: pH 6.0
digesta leaving: pH 2-3
Unlike for non-ruminants, in ruminants digesta passes to the abomasum relatively continuously throughout the day (24 h). This is because of the reticulo-rumen which holds kg of material. Thus, there is continuous secretion of gastric juice consisting of HCl and pepsin. The retention time in the abomasum is very short (1 - 3 h), whereas retention time in the reticulo-rumen is closer to h. The digesta entering the abomasum is at a pH of about 6.0 whereas the digesta leaves the abomasum at a pH of

21. This slide shows the differential development of the various compartments within the stomach. There is a rapid increase in size of the reticulo-rumen as soon as the animal starts to digest dry feed. The decrease in relative size of the abomasum is due to an increase in the other compartments - the abomasum does not get smaller.

22. Motility Mixing of digesta
passage of digesta through the reticular-omasal orifice
Movement of digesta into and out of the stomach is rather complex. The digesta enters the reticulo-rumen through the cardia.

23. Rumen motility Contraction/relaxation of reticulo-rumen
moves and mixes digesta
passage of digesta through the reticular-omasal orifice
initial contraction of the reticulum
2nd powerful contraction of reticulum
wave of contractions passing over rumen
about 60 secs
Motility of the R-R aids in mixing the newly ingested feed with that already in the rumen. Motility also contributes to rumination, eructation (expelling gas from the rumen) and passage of digesta out of the rumen to the omasum. Basic rumen motility consists of an initial contraction of the reticulum, a 2nd more powerful contraction of the reticulum, and then a wave of contractions that pass over the rumen. The entire sequence takes about a minute.

24. Contraction sequence of RR
contraction of reticulum
nd contraction of reticulum
contraction of the cranial sac
contraction of the dorsal and dorsal blind sac
wave of contraction over the ventral sac
back again
contraction of the dorsal blind sac

25. Movement of Digesta
Motility of the R-R mixes the digesta. The reticular contractions push the newly ingesta feed towards the rumen. The wave of contractions in the dorsal and then ventral sac moves the digesta towards the blind sacs. As the digesta is hydrated and digested it sinks, such that the lighter newly arrived particles are found at the top of the fibrous layer and the heavier, denser particles are found in the ventral sac. The reticulo-omasal orifice is open after the 2nd reticular contraction. Digesta that is of smaller particle size, and denser (higher specific gravity) is most likely to pass through the orifice. Longer/lighter particles are at the cardia and get ruminated.

26. Rumination - “cud-chewing” - regurgitation of digesta
- re-swallowing of liquid and fine particles
- mastication/ensalivation of bolus ( sec)
- pause (2 - 6 sec)
The process of Rumination involves:
1. Regurgitation of digesta from the reticulo-rumen into the mouth,
2. Most of the liquid and some of the fine particles, are immediately swallowed.
3. The remaining larger particles are chewed about times (about once a second),
4. The bolus is then re-swallowed.
5. There is a short pause and the cycle starts over. Rumination periods usually last from 5 mins to over an hour.

28. Eating and ruminating increases saliva secretion
helps reduce particle size of feed
promotes passage of digesta from the RR which alleviates gut fill
Both eating and ruminating are important because of: their role in increasing saliva secretion, they are involved in particle size reduction of feed which is required for digestion, and because R-R motility is highest during eating and ruminating, mastication promotes passage of digesta from the R-R which alleviates gut fill.

29. Eructation Rumen contraction forces gas to the back and then forward
Gas forced up esophagus to the trachea
GAS
GAS
GAS

30. Eating and ruminating activites can be monitored visually, as we have been doing lately. An alternative method is to monitor jaw movements. In this slide, we see a halter with a transducer positioned under the jaw. When the cow chews, her jaw presses down on the transducer and a chew is measured.

31. This is an example of the output
This is an example of the output. During rumination the jaw movements are very regular during the mastication of a bolus. During eating the jaw movements are irregular, a lot of this depending on whether the animal is picking up food into its mouth or chewing it afterwards.

32. This is an example of what the output looks like over a longer period
This is an example of what the output looks like over a longer period. A and B represent eating, C shows the end of eating and the start of rumination, D - F show rumination patterns, and G shows the end of rumination and the start of eating.

33. Esophageal Fistulae
To measure the effects of eating mastication on particle size reduction of feed we usually empty out some of the rumen contents, reach in through the fistulae, place a bag over the cardia, and collect the sample. That isn’t possible during rumination, so we have used cattle with a fistulae into the esophagus. The plug can be removed enabling collection of the digesta up or down.

34. Rumination boluses (liquid)
This is what the ‘up bolus’ and ‘down bolus’ look like. Mastication during rumination is very efficient. You can clearly see in this photo the difference in particle size before and after chewing. up
down

35. Rumination boluses (dry)
This photo shows the same thing, but the samples were dried. up
down

36. Role of rumination saliva secretion particle size reduction
2-4 x higher than during resting
particle size reduction
The extent of particle size reduction during eating depends on the type of feed. Of course, for long hay the particle size reduction during eating is extensive. We have observed that forage particles are chewed such that long-stemmed or coarse-chopped forage after ingestive mastication resembles silage in particle size, and silage resembles fine-chopped silage. Thus, feed is swallowed relatively coarse, and most of the subsequent particle size reduction occurs during ruminative chewing.

37. Very little particle size reduction occurs during digestion per se
Very little particle size reduction occurs during digestion per se. However, particles get thinner due to digestion (but not shorter), and this can result in them breaking during motility of the reticulo-rumen.

38. We have measured the time spent ruminating
We have measured the time spent ruminating. Cows spend about h/d eating - 5 h/d would be typical for a high producing dairy cow, while 8 h/d would be typical of grazing cows or cows fed a low quality all-forage diet. Cows ruminate longer than they eat, ranging from about 3 h/d for feedlot cattle fed a low-fiber diet with highly processed grain. Dairy cows ruminate about h/d. Both eating and ruminating time increase as fiber intake increases, up to a maximum of about 18 h/d. This is the physiological limit to mastication. In addition to the amount of fiber consumed, particle size of the forage affects time spent eating and ruminating.

39. Chewing activity of dairy cows
Eating
Ruminating 10
alfalfa hay
orchardgrass
corn silage 8 6
h/d
Here you see the eating and ruminating time of cows fed increasing levels of forage fiber from either long-stemmed alfalfa hay, long-stemmed orchard grass hay, or chopped corn silage. At given fiber intake, less time was spent chewing corn silage than long hay, because less particle size reduction was required for the corn silage. However, time spent ruminating was similar for all forages. This suggests that the particle size of the swallowed forages was probably similar for the hay and the silage. It is only when the particle size of the original feed is finer than the particle size of the typical eating bolus, that rumination time is reduced. This is the approach we are using to measure adequate particle length of feeds. 4 2 10 15 20 25 30 35 40 10 15 20 25 30 35 40
NDF-f (% DMI)

40. Rumination by feedlot cattle
Medium
fine 75
Coarse 86
Medium 81
Fine 69
8.1
13.6
Diets contained 10% silage (DM basis)
9.3
15.8
6.8
13.3
Ruminating, h/d
Total chewing, h/d
Diets contained 5% forage (DM basis)
Ruminating, h/d
Total chewing, h/d
9.4
16.1
Another fallacy is that feedlot cattle don’t ruminate. We have shown that this is not the case at all. When we fed whole grain without forage, cattle ruminated between h/d depending on the source of grain. Here we see that cattle fed 5% (DM basis) forage ruminated h/d. Doubling the forage to 10%, doubled the rumination time to about 9 h/d. However, reducing the particle size of the grain by processing it more finely (without decreasing forage intake) substantially reduced ruminating time. This is an important difference between dairy and feedlot cattle. Coarser grain is desirable for feedlot cattle because the grain itself acts as ‘effective fibre’ causing the cattle to ruminate. With dairy cattle, reducing the particle size of grain doesn’t decrease rumination, because the rumination is mostly due to the high forage intake.
Silage
Straw

41. The greatest portion of eating occurs during the day, while the greatest portion of rumination occurs at night. But animals do eat at night and ruminate during the day (both dairy cows and feedlot cattle).

45. Salivation lubrication of feed for swallowing
passage of feed through the GI tract
buffers (bicarbonate, phosphate) acids produced during digestion
enzymic activity
no amylase in ruminants
pregastric esterase or lipase (preruminant)
nutrients for microbes: mucin, P Mg, Cl
anti-frothing properties (bloat)
There are a number of functions of salivation by cattle including:

46. Salivary glands Parotid saliva: ~ 50 % of total saliva
There a number of salivary glands, and each type is paired on each side of the head. The parotid glands are of course the most important contributing % of the total saliva.
Parotid saliva: ~ 50 % of total saliva

47. Amount of Saliva parotid cannulation re-entrant parotid cannulation
total collection of saliva
only one duct
re-entrant parotid cannulation
collection at the cardia using a bag
requires partial rumen emptying
In terms of quantifying salivary secretion several techniques have been used. You can cannulate the parotid gland and collect the saliva per unit time. This approach is not very quantitative as the animal then eats on the other side of its mouth. Several groups have developed re-entrant cannulation techniques. Collecting masticated feed or saliva at the cardia through the rumen cannulae can be used during eating and resting, but this doesn’t work for rumination as the rumen has to be partially emptied.

48. Sheep Do animals that eat more slowly produce more saliva ?
Others have shown that changing the form of forage can affect the eating rate of the feed, but the salivation rate per minute is relatively constant. Thus more saliva is added during eating to feeds that take longer to eat. To my knowledge no one has looked at whether animals that eat more slowly produce more saliva, but this would be a logical conclusion. This may help to explain the tremendous variation in rumen pH among animals fed the same diet.
Do animals that eat more slowly produce more saliva ?

50. In this study with sheep the increase in rumen pH corresponded to rumination periods. With cattle fed mixed diets, rumen pH is usually lower than shown here because of extensive fermentation of the feed. Thus, it is much more difficult to see a clear cut relationship between rumination and rumen pH.

51. About 80% of the energy supplied to cattle is in the form of volatile fatty acids resulting from digestion of carbohydrates in the rumen. The main VFAs are acetate, propionate, and butyrate. The proportions of the individual VFA produced vary with the composition of the diet. Generally as the amount of forage in the diet decreases, acetate proportion decreases and propionate increases.

52. VFA
The change in the proportion of acetate and propionate reflect the change in the species of microflora that are prevalent at differing rumen pH. For example, as more starch is included in the diet, the pH in the rumen decreases, favoring a shift in microflora from fibrolytic species to those that utilize starch.

53. Rumen pH feed digestion chewing particle length VFA production saliva
Putting the picture together, we have feed digestion that results in the production of VFA which are absorbed through the rumen wall or passed out of the rumen with digesta. The process of eating and rumination results in tremendous quantities of saliva. The balance between the VFA and saliva should result in a pH greater than 6.0
saliva
chewing
particle length

54. Effects of level of fiber in the diet on ruminal pH of dairy cows
However, this is not always the case particularly with high producing animals fed fermentable feed for maximum intake. Generally, the ruminal pH is high before feeding (6.5). After the feed is consumed and fermentation begins, the pH is lowered. The extent of the decline is proportional to the amount of starch or OM fermented. This slide shows the typical pH pattern for dairy cows fed 2 x daily diets that range in fiber (measured as NDF) content. In vitro studies have shown that fiber digestion decreases at pH <6.2.

56. It is important to understand that for cattle fed a particular diet, there is substantial variation in pH among the various animals. That is why only some animals may experience sub-clinical acidosis (defined as pH <5.8), while other animals maintain pH on the same diet.

57. The same pattern is observed for feedlot cattle, except in this case feed was offered once daily. The line represent the mean for cattle fed the various diets.

58. This slide shows that salivary flow increased during eating and ruminating. It’s usually estimated that salivation is times higher during eating and 2.5 times higher during rumination compared with during resting, thus diets that stimulate maximum chewing time offer the greatest buffering potential.

59. This graph shows the pH profiles of 4 dairy fows fed 4 different diets
This graph shows the pH profiles of 4 dairy fows fed 4 different diets. Cow 165 maintained the highest pH regardless of diet, while pH tended to be low regardless of diet for cow 184.