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Production Resources | Forages & Grains
Diet Selection & Grazing Behavior

by Kathy Soder, USDA-ARS, University Park, PA

Added October 1, 2009. When we turn cows out into a new paddock, we’re really presenting them with a wide variety of choices, not unlike our own trips to a food buffet. While we may have planted 2, 3 or more forage species in the pasture, in reality, there could be well more than 20 plant species from which to choose. Granted, many of these species may be considered ‘weeds’, but many weed species are as or more nutritious than many of our ‘desirable’ forage species. And cows know it. But how do they know?

Cows learn in a variety of ways. First, they learn from their dams. Hmmm, think of the implications of raising calves from birth in individual hutches. What are they learning about grazing behavior
in solitary confinement, away from any education they may have received from their dam? For these reasons and others, some graziers are starting to raise calves on pasture, whether it be mob fed milk on pasture, or raised with a ‘nurse’ cow which might the biological dam, or an adopted ‘nurse’ cow. Not only do these calves benefit from the exercise, but they also learn a lot about social behavior, including how to graze, which will serve them well when they become productive members of your milking herd.

Cows learn from peers, and this can be an important aspect of grazing management if, for example, you are trying to get newly purchased cows acclimated to your farm/forages. Putting some of your ‘native’ cows in with them can help them adjust quicker (as long as biosecurity concerns can be addressed). If you turn a new group of young heifers out on pasture for the first time after being raised in the barn, put a few older animals in with them to ‘teach them the ropes’.

Cows also learn by trial and error, just like us. If a cow consumes a novel (new to her) plant and becomes ill after consuming it, she will likely avoid that plant, and in addition, teach her offspring to avoid that plant. This has been experimentally proven, even in feeds such as alfalfa or corn that are ‘safe’ for cows. If the feed is novel (the cow has never seen it before), and she becomes ill (artificially
induced illness in research) after consuming that novel feed, even if only once, she will avoid that feed. That is known as post-ingestive feedback. We experience post-ingestive feedback all the time….think about the positive post-ingestive feedback we receive from the consumption of chocolate, sugar, or alcoholic beverages. However, if you’ve ever become violently ill after eating
a certain food, even if you know that food didn’t make you sick, the sight or smell of that food will usually not be appetizing for some time. That is post-ingestive feedback.

In addition to learned behavior, we as farm managers can influence grazing behavior of cows based on our management decisions. Research in England showed that cows (and sheep) prefer clover in the morning, and grass in the afternoon. While the exact reason for this has yet to be determined, several theories include: 1) cows are ‘hungrier’ in the morning and can consume clover at a faster rate than grass to fill them up quicker; 2) clover is more palatable (it ‘tastes’ better) than grass; and 3) clover has more nutrients per bite than grass. If the above are true, why do cows bother switching to grass later in the day? Again, theories include: 1) cows may feel a bit of bloat from clover consumption which can be mediated by consuming a higher fiber feed (grass); 2) the sugar content of grasses tends to be greater in the afternoon, perhaps making it more palatable; 3) grasses tend to stay in the rumen longer and also stimulate more cud chewing, which may be an important factor during the night, when cows typically aren’t grazing as much.

Work at our USDA-ARS lab showed that ‘ruminal fill’, or how ‘hungry’ the cow is, can affect grazing behavior. We experimentally emptied rumens of cows, then re-filled them with either 0%, 33%, 66%, or 100% of the rumen contents. We then offered the cows micro-swards of grass pasture, and measured their bite dimensions (bite mass, bite depth, bite width). Not surprisingly, the ‘empty’ cow (0% ruminal fill) had greater bite mass (took a bigger bite). However, what was surprising was how the dimensions changed- the empty cow (on the left of Figure 1) took a very shallow, wide bite from the canopy. The ‘full’ cow (100% ruminal fill) took a much narrower, deeper bite into the canopy.

What does this mean? If we look at plant physical structure, the most nutritious part of the plant is towards the tip, where the youngest growth is. The least nutritious part of the plant is closer to the ground, where the higher-fiber stem and dead material are. The ‘empty’ cow was receiving hormonal feedback (via the hormone ghrelin, the same hormone that stimulates your appetite and makes your stomach growl) that she needed to harvest nutrients quickly. So she harvested only the ‘best’ of the plant- that is, she only skimmed the top of the plant to harvest the most nutritious bits, and avoided the stem that was deeper in the canopy. The ‘full’ cow was not as particular since she was not receiving that same feedback (low ghrelin levels), therefore she did not avoid the stem, biting deeper into the pasture canopy.

What can we learn from this? Our feeding and her management strategies, including when and what we feed our cows in the barn, can impact grazing behavior of cows. We must combine our farm goals with what is known about grazing behavior to decide when and how to feed our cows. Pasturing cows directly after milking may impact grazing behavior differently than if they’re fed their conserved feeds prior to being turned out on pasture. Pasturing cows only at night may impact their grazing behavior due to preference changes throughout the day. If pasture utilization is to be maximized, or if we’re dealing with a certain pasture quality, we may need to be flexible in what and when we supplement our cows to optimize their utilization of our pastures.

What does this mean? If we look at plant physical structure, the most nutritious part of the plant is towards the tip, where the youngest growth is. The least nutritious part of the plant is closer to the ground, where the higher-fiber stem and dead material are. The ‘empty’ cow was receiving hormonal feedback (via the hormone ghrelin, the same hormone that stimulates your appetite and makes your stomach growl) that she needed to harvest nutrients quickly. So she harvested only the ‘best’ of the plant- that is, she only skimmed the top of the plant to harvest the most nutritious bits, and avoided the stem that was deeper in the canopy. The ‘full’ cow was not as particular since she was not receiving that same feedback (low ghrelin levels), therefore she did not avoid the stem, biting deeper into the pasture canopy.

What can we learn from this? Our feeding and her management strategies, including when and what we feed our cows in the barn, can impact grazing behavior of cows. We must combine our farm goals with what is known about grazing behavior to decide when and how to feed our cows. Pasturing cows directly after milking may impact grazing behavior differently than if they’re fed their conserved feeds prior to being turned out on pasture. Pasturing cows only at night may impact their grazing behavior due to preference changes throughout the day. If pasture utilization
is to be maximized, or if we’re dealing with a certain pasture quality, we may need to be flexible in what and when we supplement our cows to optimize their utilization of our pastures.

 

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