How long and when should I graze my cattle?
Linking Pasture and Animal Processes
by Pablo Gregorini, USDA-ARS Pasture Systems and Watershed Management Research Unit, University Park, PA
Research has shown that cattle concentrate grazing during the evening. This coincides with the highest sugar, digestibility and less fiber concentrations in the herbage, therefore researchers have theorized that cattle concentrate their largest grazing meal when the pasture is most ‘nutritious’. Compared with turning cows onto fresh pasture in the morning, offering fresh pasture in the afternoon increases duration and intensity of evening grazing bouts and pasture intake at that time of day, which has been shown to improve animal performance. Because pasture intake rate is positively associated with hunger, pasture intake during the evening may not yet be maximized.
This fact led an Argentinean research team specialized in grazing management (National University of La Plata) to assess the impact of morning fasting periods by offering beef heifers fresh pasture in the afternoon, then evaluating evening eating, rumination, and idling times, as well as the number of bites while grazing, and pasture intake and performance of the heifers grazing a pasture of annual ryegrass. In that experiment, this research team allocated a fresh pasture each day at 2:30 pm, either with or without a 8-hour morning fast. When heifers were fasted prior to receiving fresh pasture, grazing time during evening hours increased by 20 min while total daily grazing time decreased by 81 min. Idling time increased along the day by 51 min, but as a consequence of treatment, it was concentrated during the morning. Rumination time was the same for both grazing managements, 61 min. The distribution of these behavioral activities can be seen in the next figure.
The intensity (forage consumed per unit of time) of grazing appeared to be increased when heifers were fasted before being turned onto the fresh pasture. The number of bites per unit of time was higher in morning fasted heifers during evening hours (non-fasted 54 vs. fasted 62 bites/ min). Despite this variation in grazing behavior, animal performance and pasture intake did not differ. The average daily weight and body condition score (1-9 scale) gain were 1.45 lb and 0.0135 points per day. Pasture intake per day (dry matter basis) was 9.7 lb (2.31 % of heifers live weight). According to these results, fasted heifers consumed the same amount of pasture in one third of the time compared to non-fasted. Consequently, a strategically planned morning fasting may generate longer more intense evening grazing bouts, increasing the intake of higher nutritive pasture, resulting in equal cattle performance with shorter grazing sessions.
Potential implications of this kind of management
The challenge facing graziers is to increase the efficiency of nutrients harvested by cattle, as well as to improve pasture production. Several works demonstrate that cattle trampling reduce pasture production, mainly because of physical plant injuries, soil compaction and reduction in fertilizer and water movement in the soil. Plants on compacted soils develop more roots at shallower depths and become susceptible to dry weather. Consequently, if fasting prior to afternoon pasture allocations enables producers to reduce residence time on pasture, future herbage production of such pasture would improve. Moreover, previous works of Gregorini at University of Arkansas have shown that ‘hunger’ reduce selectivity and increase the efficiency of pasture utilization. In conclusion, short and intensive grazing sessions during the evening seem to be enough.