Humans have different tastes in salad. Some people prefer just lettuce and a little oil as ingredients; others prefer to add cucumber, tomato, spinach, onion, avocado and black olive, and then add olive oil or a salad dressing. Some prefer all these ingredients but placed separately on the plate. Cattle approach their feed in a similar way. They don’t like single ingredients; they would rather have a mixed or complex diet.
If ruminants have access to different forage species, they always choose a mixed diet, although they will show partial preference for certain forages. The question is, If they require only one of those forages to meet their nutrient requirements, why do they pick a “mixed salad”? Some of these cattle behaviors when grazing different forages have been studied by scientists with the aim of understanding the underlying mechanisms of animal-plant relationships and with the objective of increasing the efficiency and profitability of animal farming.
The ability of the animal to select or exercise free choice is sensible and provides an evolutionary advantage. Preference has been defined as the ability of the animal to choose without physical restriction of access any of the diet components offered. It is best demonstrated by animals eating a certain proportion of grass and clover from monoculture pastures that are separated so the animal can exhibit free choice for either forage.
Preference is not the same as selection. Selection can be defined as the preference of the animal modified by physical constraints to access at least some of the components offered. A pasture where a grass and a legume are planted together causes the animal to search for the preferred ingredient of the diet, in this case, the legume. The barrier is the grass, so cattle express selection for what they want. Selection is demonstrated by a ruminant grazing a pasture composed of a mix of, for example, white clover (a legume) and ryegrass (a grass). In normal conditions, either of the two plants would meet the dietary requirement of the animal; however, in this type of pasture, the white clover commonly comprises 10 to 20 percent of the total available dry matter of the forage. It has been proven that cattle have partial preference for legumes. They would search for the white clover from within the ryegrass to obtain the “balanced” diet they prefer. Consequently, a typical diet is composed of 50 percent or more clover (or other legume) of the total forage dry matter consumed.
The next question is, Will this response change if we add a mix of legumes? To answer it, crossbred beef heifer calves with an average body weight at the beginning of the grazing period of 525 pounds were offered different grazing options: annual ryegrass only; a clover mix only of white, red and berseem clovers; a mixed pasture of annual ryegrass and the same clover mix; and adjacent areas of equal size of annual ryegrass or the clover mix. To study calves’ grazing behavior, one heifer per group was fitted with a cattle pedometer. Video cameras were used to study the cattle during the day.
Heifers placed on pastures with mixed annual ryegrass and clovers or in pastures with separated ryegrass and clover areas gained 2.77 pounds per day and produced 245 pounds per acre. Calves grazing only annual ryegrass or only clovers alone gained 2.57 pounds per day and produced 228 pounds per acre. Calves in separate but adjacent pastures spent less time grazing because the two major diet components, mixed clovers and annual ryegrass, were in distinguishable portions of the paddock (Table 1), thus eliminating the need for excessive searching for different species. The proportion of time they spent grazing each pasture type is shown in Figures 1 and 2.
The calves grazing the annual ryegrass and clovers mix walked more, searching for the appropriate balance between annual ryegrass and clover (Table 1). As averaged throughout the grazing season, calves grazed longer on the clover mix (thus ate more) than on annual ryegrass (Figure 1), and they spent more time grazing the clover mix in the morning hours (75 percent) than in the afternoon (60 percent). This means that preference for annual ryegrass increased in the afternoon (25 percent in the morning versus 40 percent in the afternoon). One theory to explain this result is that eating fiberrich forages (annual ryegrass in this case) in the afternoon requires more time to digest in the rumen, helping cattle to maintain gut fill and decrea es the need for them to graze at night, avoiding possible predators. In the present experiment, however, there were no differences in fiber content (expressed as neutral and acid detergent fiber) between pasture types (Table 2).
Daytime patterns of preference in ruminants have also been attributed to changes in forage carbohydrate (sugar) concentrations as day progresses from dawn to dusk. Tests showed differences in water-soluble carbohydrate concentrations between pasture types (Table 2) and also between forage samples taken in the morning and afternoon hours. Annual ryegrass had a greater concentration of water-soluble carbohydrates in the afternoon (22.1 percent of the dry matter) than in the morning hours (12.9 percent of the dry matter). It was greater than the concentration in the clover mix – 9.9 percent of dry matter in the morning and 6.1 percent of dry matter in the afternoon. These data may explain the increase in time spent on annual ryegrass in the afternoon because it was “sweeter” (more sugar) than in the morning hours, although heifers still showed a partial preference for clover mix (75 and 60 percent of the time, Figure 1). Throughout the day, there was a clear trend to decrease the time spent grazing on the clover mix (Figure 2).
There was also a clear disparity between the average time on clover mix throughout the grazing period and that for the last 30 days (Figure 2). In the latter, the time spent grazing on clover mix was less than 50 percent during the major afternoon grazing period (after 3 o’clock in the afternoon), probably associated with the lower water-soluble carbohydrate concentrations of the clover mix, but also because of the changes in botanical composition (Figures 3A and 3B). From day 75 of grazing until the end of the grazing period, the proportion of berseem clover decreased significantly, drastically reducing the amount of forage available. Red and white clover started to increase their contribution to the dry matter produced (Figure 3A) at that point, but it was not abundant.
Calves grazing different pasture configurations (mixed or adjacent) gained the same amount of body weight, but their grazing behavior was affected. Animals grazing adjacent pastures were offered the opportunity to select their diet at the time of the day they wanted, which gave them the chance to walk less than those grazing other pasture types where calves needed to spend time searching for the missing component of their diet (grass or legume). Differences in the concentration of water-soluble carbohydrates between grass and legumes may explain the different proportion of time that calves spent grazing on adjacent pastures of annual ryegrass or clover mix; however, during the last 30 days of the grazing season, the reduction in the proportion of berseem clover also influenced the proportion of time that calves grazed on annual ryegrass or clover mix.
Guillermo Scaglia is an associate professor at the Iberia Research Station, Jeanerette, La.
(This article was published in the 2014 winter issue of Louisiana Agriculture magazine.)