Graziers, your livestock are telling you how to increase pasture intake

We know that performance of dairy cows and growing animals is highly dependent on feed intake. Did you know that the grazing behavior of your livestock can show you how to manage grazing to maximize pasture intake?

Recent work by university scientists in Brazil sheds light on how we can manage livestock grazing to maximize dry matter intake and animal performance on pasture. Dr. Paulo Carvalho has led this work at the federal university in the state of Rio Grande do Sul along with collaboration with Dr. Anibal de Moraes at the federal university in Parana, both in southern Brazil.

Livestock grazing intake

Carvalho’s research on animal grazing behavior has demonstrated what regulates intake rate and animal performance on pastures.

His team spent time visually observing and recording animal behavior on pasture. Their team documented exact changes in the weight of animals for defined periods of time on pasture. They even collected all urine and feces in order to accurately measure animal weight change during grazing events. Computerized instruments were attached to the necks of the animals to record the number of bites per unit of time and the time spent chewing.

From all that data, Dr. Carvalho’s team was able to directly and accurately calculate the actual amount of forage dry matter harvested by animals during grazing. The results are truly intriguing.

Tipping point

First of all, Carvalho’s team discovered a tipping point during grazing at which the animal intake rate declines sharply. Once the animals grazed down the pasture sward height by 40 to 50% of the initial height, the forage mass per bite began to decline sharply. At this point of 40 to 50% reduction from the initial pasture height, the number of bites per second began to drop dramatically. The result was that the animal spent more time foraging to obtain the desired meal.

This point also corresponded to when the percentage of leaves in the sward began to decline rapidly and stem mass increased. So 40 to 50% reduction in initial pasture height represents when intake rate begins to decline sharply, and this holds true across diverse grass species.

Carvalho demonstrated this in oat, annual ryegrass, Bermudagrass, and sorghum-sudangrass.

When intake declines

So you may ask, does the initial pasture height influence the point when there is a decline in forage intake by the animal during the grazing event? The answer is “No.” Animal intake declines once the pasture height is grazed down by 40 to 50% regardless of what the pasture height was initially when animals were placed in the paddock.

Carvalho also found that animals, regardless of species, will adjust their bite size to harvest about 50% of the existing pasture height in front of them. So when an animal enters a new paddock, each bite consists of 50% of the initial sward height present. The lower 50% of the initial sward height will become the second layer or horizon grazed, and the animal will remove only 50% of that.

When to graze

There is an optimal initial sward height to maximize animal intake, and that height varies by forage species. For the forages tested, to maximize intake of grazing animals do the following:

• Oat — Start grazing at 11 to 12 inch sward height and graze down to 6 to 7 inches.
• Annual ryegrass — Start grazing at 7 inches and graze down to 4 inches.
• Bermudagrass — Start grazing at 7 to 8 inches and graze down to 4 to 5 inches.
• Sorghum-sudangrass — Start grazing at 20 inches and graze down to 12 inches.

(Source: Dr. Paulo Carvalho)

Think about it

Let’s consider a pasture that is 20 inches tall when animals are first placed in the paddock. The animals will graze off the top 10 inches per bite across the paddock, on average.

Once most of that first 10-inch horizon is gone, the animal will start grazing the lower 10-inch horizon that remains, but it will adjust its bite size to remove only the top 5 inches of that next layer. So the animal always removes 50% of the sward height in front of it.

Basically, the animals graze most rapidly and eat the most during their first “meal” that is represented by the top half of pasture height present when the animal is first placed into a paddock.

If the animal is forced to graze the second horizon after removing the first half of the initial pasture height, then its grazing efficiency declines dramatically.

Height and intake

The second important discovery made by Dr. Carvalho was that there is an optimal pasture height at which forage intake is maximized — and that height does differ by forage species.

For annual ryegrass, animals had the greatest forage intake when the initial sward height was 7 inches tall. For oat, the optimal sward height to maximize animal intake rate was 11-12 inches. For bermudagrass, it was 7- to 8-inch sward height, and for sorghum-sudangrass it was 20 inches.

But regardless of the initial sward height, animal intake is maximized while the pasture is within 50 to 60% of its initial height. In other words, intake is maximized until about 40% of the initial height is removed.

So the bottom line to all this is that the entry and exit points in rotational stocking systems can be managed to optimize forage intake and animal performance on pastures.

Plans are being made by the Brazilian researchers to evaluate the optimal sward heights for tall fescue and orchardgrass. I will share those results in this column when they come available.

Bottom line

When you remove about 40% of the initial pasture height on average, about 25% of the area in the paddock will appear to not have been touched by the animal. If you force the animals to stay longer, they will begin grazing the next horizon, which will decrease their efficiency. So it is best to move high producing animals out of the paddock once they graze off 40% of the initial sward height.

Try this on your farm and watch the bulk tank (if dairy) or animal weight gains (for growing animals).

Spend some time measuring the initial sward height in paddocks and remove animals when the sward height has been reduced by 40%, leaving about 25% of the total area in the paddock not grazed off. Then watch the regrowth in those paddocks compared with grazing them all the way down as usual.

You might be surprised by what you see.