Pasture & Range: March 2000 Other Pasture & Range Articles

Small grains such as wheat, rye and ryegrass are often used as forage during cool season months for stocker cattle development. These forages are costly to establish, but are efficiently converted into gain by the use of growing animals. A substantial number of livestock producers supplement their cowherd with winter pasture while being full-fed hay simultaneously. In these feeding regimes, mature cattle have anything from full to some form of restricted access to winter pasture. Restricted access or limit grazing winter pasture is used as a means of protein supplementation. Producers frequently ask, "How long and how often should I allow cattle to graze winter pasture to meet their nutrient requirements?" We initiated research to gain some insight into this dilemma.

The study's first phase began in 1998. Three rumen-fistulated steers (steers surgically operated on to provide the researcher direct access to the rumen) of mature size were placed into three separate winter pasture paddocks of uniform size. The steers were observed grazing at time intervals of 15, 30 and 45 minutes for three consecutive days per trial. Sampling was performed using the complete rumen evacuation method. Dry matter intake, dietary crude protein, dietary total digestible nutrients, and bite rate, along with quantitative and qualitative pasture measures were the variables for which data was collected. Three trials were scheduled for each of two seasons, plus one preliminary trial. The reported results represent data from the preliminary trial and the three trials from the first season, the winter of 1998-99. Pasture summary and diet statistics are listed in table 1. Even though pasture height and standing crop determinations were variable, pasture quality was less variable. Dietary crude protein was consistently higher than found in pasture samples. Cattle tend to be selective grazers even when forage quality is high.

Protein supplements are fed on a pound per head basis and are categorized according to their protein content. A 38 percent cube contains 0.38 pounds of supplemental protein per pound of cubes fed. If two pounds of 38 percent cubes were needed per day per head to meet nutritional needs of the cowherd, then each cow would receive 0.76 pounds of protein. What does this represent in terms of limit grazing winter pasture? Since data was collected at each of the three grazing intervals (15, 30, and 45 minutes), the crude protein intake at each interval was determined. Table 2 summarizes the crude protein intake per grazing interval. Bite rate was faster during the initial 15 minutes of grazing as compared to the other grazing intervals. Therefore, crude protein intake during the first 15 minute grazing interval was twice as great as the two successive intervals. According to the data, mature cattle can consume about 0.4 pounds of crude protein in the first 15 minutes of grazing and an additional 0.2 pounds during each of the next two 15 minute grazing intervals. In 45 minutes, a cow can consume approximately 0.8 pounds of crude protein, which is equivalent to 0.76 pounds of crude protein found in two pounds of a 38 percent cube. Again, these results are preliminary.

Table 2. P Intake by Grazing Interval (#DM)
Trial Date	15 min.	30 min.	45 min.
Mar. 98	0.28	0.50	0.81
Dec. 98	0.42	0.46	0.63
Mar. 99	0.48	0.72	0.87
Apr. 99	0.49	0.68	0.93
Average	0.42	0.60	0.81
Other Variables (avg.)
DMI* (lb.)	1.41	1.92	2.72
TDNI* (lb.)	1.02	1.42	2.01
Bite Count	712	1166	1533
Bite Count/interval	47.5/min.	30.3/min.	34.5/min.
*DMI= Dry Matter Intake *TDNI= Total Digestible Nutrient Intake

Actual intake of small grain pasture by mature cattle has not been thoroughly evaluated. A better understanding between the relationship between grazing interval and dietary intake would allow producers to more precisely manage forage intake relative to livestock requirements. The purpose of the study is to develop guidelines for limit grazing mature livestock while preventing inefficiencies in winter feeding programs when using small grain pasture as a source of protein supplement.

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