In a year with plentiful rainfall, pastures remain lush and continue to grow throughout the summer. It is always encouraging to see green grass in August, rather than having all fields brown and needing to feed hay. Pasture gains in these wet years are often a little disappointing. One would think that with all the lush grass and high quality feed available all season, gains should be excellent, but this is not necessarily the case.

To understand why this happens we need to look at how an animal eats on pasture. Cattle bite at about 15 bites per minute for 6–10 hours per day. Body fill is the main factor determining when they quit eating. In a year with adequate rainfall, the dry matter (DM) content of the grass is lower — likely in the 15–20% range. In a drier year, the grass may have a dry matter content of 20–25%. If an animal grazes for 8 hours per day at 15 bites per minute, this represents 7,200 bites each day (15 bites/minute x 60 minutes x 8 hours).

Pasture moisture limits dry matter intake

As an example, a 400 kg animal on pasture requiring 2.5% of body weight in dry matter intake for maximum growth would need to consume 10 kg of dry matter per day. If each bite size is a typical 7 grams and this animal is going to take 7,200 bites per day, then it will consume 50.4 kg of pasture (7,200 bites x 7 grams).

If the pasture is 20% dry matter (typical of a normal year), this 50.4 kg represents 10.8 kg of dry matter and the animal has met their nutritional needs.

If this pasture was lush and had 15% dry matter (typical of a wet year), then our beast would consume only 7.5 kg of dry matter (50.4 kg x 15% DM). This falls short of its dietary needs. Under this scenario the animal needs to consume 66.6 kg of pasture to meet their optimum needs. This means either eating for longer (more bites), or not meeting its energy needs and having less than optimum growth.

In a dry year, when the pasture would have 25% dry matter, this same animal would consume 12.6 kg of dry matter (50.4 kg x 25% DM), well above the requirement of 10 kgs. This animal could either graze for fewer hours and still meet its requirements, or graze for the same time and have exceptional gains.

In a wet year it takes more hours of grazing to meet the dietary needs than it does in a dry year, assuming adequate forage is available.

An analogy

To see this in another way, it is like sitting down to a meal that is a big bowl of soup. Across the table from you is a person with a bowl of stew. You both have the same sized spoon. Who is going to feel full or satisfied first? The person eating stew will. Can you get enough nutrition from the soup? Yes, but only if there is a second or third bowl offered and you have longer to eat your meal.