Laboratory 1
THE EQUINE DIGESTIVE SYSTEM

Source: Rachel Monticelli-Turner.

Introduction:

Understanding the anatomical organization and function of the equine digestive system is very important in laying a foundation of knowledge about the nature of feeding horses. In this laboratory exercise the student will review the locations and functions of the organs of digestion, paying particular attention to where the various nutrients are digested and absorbed. This base of knowledge will foster an appreciation for why horses utilize certain types of feeds more efficiently and why horses are prone to various digestive-related disorders.

Objectives:

When finished with the material from this laboratory, the student should be able to:

1. Identify all organs that relate both directly and indirectly to the digestive system of the horse.
2. Define the following terms:
   1. digestion
   2. absorption
   3. metabolism
   4. herbivore
   5. nonruminant
   6. cecal fermenter
   7. ruminant
   8. prehension
   9. mastication
   10. colic
   11. choke.
3. List sizes, capacities, and functions of the organs of the equine digestive tract.
4. List the locations of digestion and absorption for the six classes of nutrients.
5. Describe the types and arrangement of teeth in the horse’s oral cavity and how their shape changes over time. Relate the changes to approximate ages of the horse.
6. Identify similarities and differences of the equine digestive system to both the simple and ruminant digestive systems.

NOTES:

Question for further discussion:

1. Why is the horse prone to digestive disorders such as colic and choke? Suggest feeding strategies that would avoid these situations.

General overview:

As a nonruminant herbivore, the horse is designed to live on a diet of plants, the majority of which are forages. Wild horses graze for 12 to 16 hours per day, consuming small amounts at any one time. Hence, the nature of the horse’s digestive tract is to accommodate small, frequent intakes. The horse’s teeth wear more evenly and consistently when the animal is able to chew coarse plant material for several hours each day. While the majority of horses on the earth today are no longer wild, they still thrive best when their feeding program is organized to provide small meals, primarily consisting of quality forage, throughout the day.

A thorough understanding of the horse’s digestive system is paramount to organizing an appropriate feeding regimen. It also creates the base for understanding why the horse is prone to certain digestive disorders. Keep the size of the mature horse in mind while exploring the numerous organs of digestion, their sizes and their functions throughout this lab.

Ruminant animals such as cows can digest quite efficiently by fermenting feed in a multi-chambered stomach. Most nonruminants lack this fermentation ability in their foregut, and are therefore called “simple stomached”. They may either lack fermentation ability (like humans and pigs) or they may be “cecal fermenters” (like horses) where fermentation happens in the hindgut.

NOTES:

THE EQUINE DIGESTIVE TRACT: PRIMARY AND SECONDARY ORGANS OF DIGESTION

A. Primary organs of digestion: The upper digestive tract

The following outline provides information pertaining to the primary organs of digestion (feed passes through them) that are considered to be part of the upper digestive tract of the horse. Use this guide when observing the specimens and diagrams provided in the laboratory, but also note there are some areas of the outline intended for the student to complete!

1. Oral cavity
   1. Lips: well developed and muscular; important tactile properties and involved with prehension of feeds
   2. Teeth: involved with prehension and mastication of feeds; deciduous (baby or milk) teeth erupt first and are then replaced by permanent teeth

      Figure 1.1 Teeth of an adult horse.

      Source: Rachel Monticelli-Turner.

   3. Identify each of the following types of teeth (Pence, 2002):
      1. Incisors (I) or front teeth: 6 total (3 per side) on both top and bottom jaws of mature horse, named in pairs for their location: central (I1), intermediate (I2), corner (I3)
      2. Cheek teeth or grinders:
         1. Premolars (P): 6–8 on both top and bottom (3–4 per side), depending on whether wolf teeth are present. They are named for their location; P1, P2, P3, and P4. P1 is also known as the wolf tooth. (More information on wolf teeth can be found on the next page in the sidebar.)
         2. Molars (M): 6 on both top and bottom (3 per each side)
      3. Canines (C): a single canine tooth may be located in the diastema (the space between the incisors and the premolars, also called the bar) and it may be in the top and/or the bottom jaw; canines are more commonly associated with stallions or geldings; a horse might not have any canine teeth or it may have as many as four canines; other names for the canines include: eyeteeth, bridle teeth, tusks, or tushes.
      4. Parts of the equine tooth (Frandson, Wilke & Fails, 2009): locate each part on specimens or diagrams provided:
         1. root—anchors tooth in its bone socket (alveolus) and is attached to the surrounding bone by periodontium (connective tissue), creating a gomphosis (fibrous) joint
         2. crown—the portion of the tooth above the gum line
         3. neck—the space between crown and root; equine teeth do not have prominent necks and are referred to as hypsodont

             

            Dental Formula:

            The traditional method for listing the potential number of teeth in a horse is to start with the letter abbreviation of the tooth followed by the number of teeth in the upper jaw placed over the number of teeth in the lower jaw (making it look like a fraction) for half of the horse’s head. For example: The total number of incisors for the upper jaw is 6 as well as for the lower jaw. Therefore, the dental formula for only the incisors is:

            or 12 total incisors. Deciduous or “baby” teeth are abbreviated with a “D” followed by a lower case letter “i” for incisor or lower case “p” for premolar.

            Review the following dental formulas:

            Deciduous (milk) teeth

            Permanent teeth

            Figure 1.2 Sagittal section and occlusal surfaces of a permanent lower incisor tooth.

            Source: Rachel Monticelli-Turner.

         4. dentin—the mineralized substance that comprises most of a tooth’s interior; center area called dental cavity contains the dental pulp
         5. dental pulp—portion of dental cavity that includes connective tissue, nerves, blood vessels
         6. enamel—covers dentin, comprised of inorganic crystals (very hard!); its prominent folds on the grinding surfaces of hypsodont teeth are called cups
         7. cementum—thin mineral layer extending from the root and covering the crown of the tooth and filling in the infundibulum of central enamel
         8. cup (infundibulum)—the dark central area of the younger tooth created by folds of enamel on grinding surfaces, this may be found on both deciduous and permanent teeth and wears away as the horse ages
         9. dental star—this is comprised of the secondary dentin and often first appears as a line in the central incisors, then appears in the intermediate and corner incisors; found on both deciduous and permanent teeth

       

      NOTES:

      Since mares frequently lack canine teeth they can have as few as 36 teeth.

      Wolf teeth—The first premolar, if present, is called the “wolf tooth”. These are small vestigial premolars usually found in the upper jaw, but they may also be located in the lower jaw. Their shallow roots allow for a relatively easy removal by a veterinarian. This is often necessary as they will sometimes interfere with the horse’s comfort when wearing a bit.

      The incisors found in ruminants have a short crown and a more prominent neck than what is found in equines; such teeth are called brachyodont (Frandson, 2003) (Greek: brachy refers to short; hypsi refers to high).

       

      Aging a horse by its teeth: This was a common practice before the current positive identification systems (tattooing, microchipping, etc.) came into use. It is still useful in determining an approximation of age (Riegel and Hakola, 1999).

      Dental Time Line

      
      

      6 days

      eruption of central deciduous incisors

      Birth–2 wks

      eruption of deciduous premolars 2, 3, and 4

      6 wks

      eruption of intermediate deciduous incisors

      6...