In general, the more livestock eat, the more weight they gain or milk they produce. Thus, forage intake is key to animal performance. Agronomists manage for plant density and height to ensure livestock maximize intake. While plant structure is important, intake is not dictated by structure alone. Forage quality, current nutritional state, and experience also affect forage intake by livestock.
So, let’s take a look at what we know, and how we can use that for the benefit of our livestock.
First, let’s look at Intake
Daily intake can be calculated using the following equation:
Structure can dictate bite size
According to a number of research studies, bite size has the greatest effect on intake. We can maximize bite size by maintaining pastures in an immature and leafy, vegetative state, and by keeping plant height at no more than 6 – 8 inches and no less than 2 to 2.5 inches. When forage grows above 6 to 8 inches, nutritional quality declines as the proportion of stems relative to leaves increases. Bite size also decreases as animals attempt to select leaves over stems.
When forage height drops below 2.5 inches, bite size declines due to a decrease in forage availability. Livestock must spend more time grazing and increase their bite rate to eat the same amount of forage. If forage is too short, livestock cannot graze fast enough or long enough to maintain intake and performance (Kenny and Black, 1984).
Nutritional quality matters
Many believe that the rate of forage intake is fixed, and determined solely by bite size and rates of chewing and swallowing, which are determined by plant density, height, and toughness. However, the nutritional quality of forage is a key factor influencing intake rates.
For example, long wheat straw is tough, not very nutritious, and hard to bite and chew. But, when we boosted its nutritional value by giving sheep a solution of starch and water via stomach tube every time they ate it, bite size, bite rate and intake all increased.
We can also see this in pasture studies. Researchers found that cattle preferred vegetative forages to to taller more mature stands of grass because vegetative stands were higher in nutritional quality than reproductive stands. When grazing pastures with mixes of mature and vegetative grasses, the cattle increased their grazing time and biting rate of vegetative growth to maintain total intake and diet digestibility (Giane et al., 2003). Not only does this show cattle’s preference for and ability to choose more nutritious foods, but it also tells us why we sometimes see overgrazing of new growth as compared to more mature forages.
Small amounts of experience browsing or grazing a plant can mean big changes in intake rates. You can see a good example in the video below comparing the bite rate of an inexperienced goat with a goat experienced with eating stemmy black brush.
We see this in many studies. As another example, inexperienced lambs fed chopped serviceberry in boxes were compared with lambs with 30 hours experience browsing serviceberry. Experienced lambs had faster bite rates and their intake rates were 27% higher. Inexperienced lambs took larger bites than experienced lambs but could not make up for their slower bite rate. In addition, they had more difficulty nipping bites off the plant than experienced lambs (Flores et al., 1989).
We’ve also found that young animals learn foraging skills more quickly than older animals. Six-month-old goats browsing blackbrush had faster bite rates than 18-month-old goats even though both groups of goats had browsed the shrub for 30 days. In addition, after 30 days bite rates for 6-month- old goats were still increasing, whereas bite rates for 18- month-old goats had leveled off (Ortega-Reyes and Provenza, 1993a).
To some degree, skills acquired by lambs on one type of plant – grass or shrub – are specific to that plant form. Lambs experienced browsing shrubs are more efficient at harvesting shrubs than lambs experienced grazing grass, and vice versa. Nevertheless, skills transfer from one shrub to another. Goats with experience browsing blackbrush were more efficient at harvesting oak leaves than goats without browsing experience (Ortega-Reyes and Provenza, 1993b).
Intake rate is often thought to be solely dependent on plant structure. However, plant structure, current nutritional state of the animal, prior experience with the plant, and the acquisition of foraging skills interact to influence rates of intake. Managers can improve intake rates in their animals by:
•Keeping pastures at the correct height;
•Feeding foods in the barn that complement the nutritional composition of forages in pastures; and
•Exposing young animals to the forages they will be required to eat later in life.
You might also train your livestock to eat weeds. Kathy Voth has found that animals trained to eat one weed, like Canada thistle for example, become more open minded about foods and learn more quickly how to approach unusual forages. (Plus you’ll have 43% more forage!)
Flores, E.R., F.D. Provenza, and D.F. Balph. 1989. Role of experience in the development of foraging skills of lambs browsing the shrub serviceberry. Appl. Anim. Behav. Sci. 23: 271-278.
Ginane, C., M. Petit, and P. D’Hour. 2003. How do grazing heifers choose between maturing reproductive and tall or short vegetative swards? Appl. Anim. Behav. Sci. 83:15-37
Kenney, P.A., and J.L. Black. 1984. Factors affecting diet selection by sheep. I. Potential intake rate and acceptability of food. Aust. J. Agric. Res. 35:551-63.
Ortega-Reyes, L., and F.D. Provenza. 1993a. Amount of experience and age affect the development of foraging skills of goats browsing blackbrush (Coleogyne ramosissima). Appl. Anim. Behav. Sci. 36:169-183.
Ortega-Reyes, L., and F.D. Provenza. 1993b. Experience with blackbrush affects ingestion of shrub live oak by goats. J. Anim. Sci. 71:380-383.
Villalba, J.J., and F.D. Provenza. 1999. Effects of food structure and nutritional quality and animal nutritional state on intake behaviour and food preferences of sheep. Appl. Anim. Behav. Sci. 63:145-163.
Villalba, J.J., and F.D. Provenza. 2000. Postingestive feedback from starch influences the ingestive behavior of sheep consuming wheat straw. Appl. Anim. Behav. Sci. 66:49-63.
Wickstrom, M. L., C. T. Robbins, T. A. Hanley, D. E. Spalinger, and S. M. Parish. 1984. Food intake and foraging energetics of elk and mule deer. J. Wild.