A number of years ago, I was speaking at a grazing conference to a group of dairy farmers about how their cows went about selecting what to eat from the vast array of plants found in most pastures. I had just finished telling the group how cows use their senses of sight, smell, touch, and taste coupled with the influence of post-ingestive feedback mechanisms to guide their dietary selections. I further suggested that I had personally observed a cow happily munching away on grass tops, dandelion leaves, a sprig or two of clover, and then go back and eat the bottom of the grass she had just taken the top off, essentially nubbing it to the point she had mud on her muzzle.
And out of the audience, I heard this guy say “aw that is bullshit, I have never seen my cows do anything like that.”
I can tell you, there is nothing quite like being at the front of the room with a laser pointer in hand and having someone in the audience, with a serious need for some vision correction, calling you a liar to make a person think unkind thoughts. However, once I got my blood pressure back under control and suppressed the urge to burn a small hole in the guy’s eye with my laser pointer, I asked him a simple question.
“Have you ever stood out in your pasture with a clipboard in one hand and a stop watch in the other and watched your cows eat hour after hour and day after day recording what they ate and how fast they ate it?”
As expected, he said that he had not. I then proceeded to tell the visually deficient mouth that I had, and furthermore, if I had not observed what I said I had observed, I would not be standing there telling people that I had.
As a general statement, most farmers do not have the time to spend in their pastures watching their livestock munch away hour after hour day after day recording what they observed and then sit down at their computer and run a statistical analysis on the data they have collected to see if there are differences in what their critters are eating, how much they are eating, when they are eating it, and then try to figure out what it means.
Again, as a general statement, that is a job for someone involved in research; someone with the training, education, and knowledge of experimental design, data collection, and statistical analysis and interpretation.
Thus it is not surprising that the mouth in my audience had not seen his cows do what I had described, what is surprising is that because he had not seen it, according to him, neither had I. On the other hand, I have never seen gravity…
How many times a day do you suppose a person looks but does not see, listens but does not hear, touches but does not feel, breathes but does not smell, or eats but does not taste? All I would have to do is ask my wife if I really wanted to know about me. Sometimes I don’t even have to ask…..
However, if an herbivore fails to do these things they are likely to get more than a tongue lashing. For millions of years before we came along to show them the error of their ways, the ancestors of modern livestock foraged, lips to leaves, in and among a vast array of plants seeking out the foods that most closely met their ever-changing nutritional requirements and leaving behind those that did not. As it is today, some plants were higher in nutrients and lowers in toxins, others were higher in toxins but lower in nutrients, and some plants varied in nutrient to toxin ratio by location, time of day, season, and environmental conditions. Selecting what to eat and where was, and still is, dangerous. Consuming the wrong plant at the wrong time or eating too much of one plant and not enough of another can cause sickness, malnutrition, and even death.
Herbivores have been selecting their own diets for a very long time, and it is much more than a random process where they eat whatever happens to be in the way. And despite a missed cue every now and again, they are very good at it.
They use their eyes to locate general foraging locations and sometimes even individual plants or parts of plants (not to mention keeping an eye out for things that would like to make a meal of them) their sense of smell to evaluate the hundreds of volatile gases that are given off by plants as well as the environment around the plants, their sense of touch to evaluate texture and fiber value, and their sense of taste to differentiate among flavors (sweet, salty, sour, bitter, and perhaps umami, pungent, and metallic.) And once the forage has been eaten, it is further evaluated postingestively by osmotic, mechanical, and chemical receptors residing within the animal’s gut.
Selecting what to eat is a dynamic process, and is the result of the interactions of two interrelated systems. One system is called the cognitive or voluntary system. This system uses the senses of sight, smell, touch and taste along with information learned from mom, other members of the herd or flock, and past trial and error encounters to determine what to eat or not to eat. Experiences early in life and guided by mom are extremely important in determining what any animal will chose to eat, even years later. If mom eats it and baby watches, chances are, baby will eat what mom eats.
This, of course, is only going to hold true in most, but not all situations. For example, my parents both loved to eat shellfish, scallops in particular, and could do so with no ill effect. I, on the other hand, have an allergy to shellfish. Early in my life Mom said “try this honey, you will like it.” Honey tried it and darn near died! Needless to say, while I have observed and continue to observe mom and many other folks eating scallops, my first experience with the darn things was so bad; I get nauseous just smelling them!
The other system at work explains why I do not even have to think about not eating scallops, and why some other animals do or do not eat particular foods at particular times. This system is called the affective or involuntary system. This is a subconscious process that operates without any effort on the part of the animal that links the taste or smell of a food with its postingestive (after it is eaten) consequences relative to the requirements of the animal.
Postingestive feedback is an information system that utilizes chemical, osmotic and mechanical receptors within the gut of an animal to evaluate the unique chemical content of each food ingested relative to the particular animal’s nutrient requirements. This information is fed back to the brain where decisions about the food are made.
Simplistically, if an animal eats a particular food item and shortly after feels sick, discomfort, or in some other way “not good,” the taste of this food will be paired with the discomfort, and the animal will likely shy away from or become averted to consuming this food item. On the other hand, if a food item is consumed and the animal feels satiated i.e., no ill effects or feels “good,” the animal will generally pair the flavor of this food with the feeling of satiety and develop a preference for the food item; and the item will likely become part of the preferred diet.
Within the dynamics of plant-herbivore interactions, it must be recognized that plants are a complex mixture of some chemicals that are used by animals as food i.e., carbohydrates, protein, minerals, vitamins and the like, and some chemicals that plants use to prevent themselves from becoming food i.e., alkaloids, terpenes, phenols, and other toxic compounds, and they are constantly changing in concentrations and ratios. Thus, in order to avoid over-ingesting toxins and under-ingesting nutrients, animals must be able to evaluate the nutritive value of foods as well as the toxic properties and generally avoid the nutritionally deficient, nutritionally excessive, and toxic foods, and select those that generally meet their requirements
While the voluntary and involuntary systems function as two separate systems, they are integrated through the senses of sight, smell, taste, and postingestive feedback. Animals use the involuntary system to evaluate the postingestive consequences of consuming a food, and the voluntary system to change their behavior towards the food depending on whether the postingestive feedback was pleasant or not.
Through this interactive exchange of information, animals constantly monitor the foods they consume and alter their diets in response to their own ever changing nutritional requirements and changes in the forage and foraging environment. And just because you have not seen it does not mean it does not happen.