The following article consists of excerpts from a program presented at the Grazing Lands Conservation Initiative Conference in December 2000. The information summarizes results of a research project conducted at the Noble Research Institute by Frank Motal, Robert Self, Evan Whitley, and me. I reported preliminary results in a previous News and Views article.
A substantial number of livestock producers use winter pasture to supplement their cow herd's diet while providing full-feed hay simultaneously. In these feeding regimens, mature cattle have anything from full to restricted access to winter pasture. Restricted access grazing or limit-grazing of winter pasture is a means of protein supplementation. This research is the first of its kind to study mature cattle's dietary crude protein (CP) intake from small-grain pasture. The purpose of the study is to develop limit-grazing guidelines to prevent large inefficiencies in the winter feeding program when small grain pasture is used as a protein supplement.
We characterized pasture conditions before each collection period and assessed and monitored forage height, forage production, and CP percentage.
Growing conditions for each of the three production stages as represented by collection periods (months) were considerably different for the two years. The pasture conditions for the December 1998 collection period reflect above-average growing conditions for that fall. Forage height (12.7 inches) and production (2,223 pounds per acre) for this collection period represented the highest values of the six collection periods. The data for December 1999 demonstrated good fall forage growth (forage height was 8.2 inches; production was 1,677 pounds of dry matter [DM] per acre), although not as good as that of the preceding fall. Pasture conditions for the April collection periods indicate that the early spring growing conditions for April 2000 were considerably better than that for April 1999. Forage was 5 inches taller and forage production was approximately 1,000 pounds more for April 2000 than for April 1999 (12.2 inches and 1,925 pounds per acre versus 7.0 inches and 933 pounds per acre, respectively). The March collection periods represented the minimum grazed height of the winter production period, marking the end of fall-phase production and the beginning of spring-phase production. Forage height and production for the March 1999 collection periods were 5 inches and 757 pounds per acre; for March 2000, 6.7 inches and 1,015 pounds per acre. Forage availability did not limit forage intake for the grazing steers during any collection period.
Because of the small sample size, variables were averaged across years, collection periods, and steers for each of the grazing intervals. Average steer weight was roughly 1,650 pounds. Forage intake increased from one grazing interval to the next, but not at the same rate. Forage intake (DM or DM basis) was 1.68, 2.49, and 3.22 pounds for the fifteen-, thirty-, and forty-five-minute grazing intervals, respectively. As expected, CP as DM percentage remained relatively consistent across grazing intervals, at approximately 27 percent. Crude protein intake was 0.45, 0.67, and 0.85 pound for the fifteen-, thirty-, and forty-five-minute grazing intervals, respectively.
Crude protein and forage intake of winter pasture under a limit-grazing feeding regimen was higher in the first fifteen minutes of grazing than in each of the next two fifteen-minute grazing intervals. The difference in CP intake between the fifteen- and thirty-minute grazing interval was 0.22 pound, and between the thirty- and forty-five-minute grazing interval, it was 0.18 pound. Comparatively, the CP intake from the initial fifteen-minute grazing interval was approximately twice that of either of the two successive grazing intervals.
In conclusion, if protein deficiencies can be ascertained in a winter forage supply (hay or standing forage) and nutrient requirements are known for a particular set of mature cattle, then an appropriate grazing interval can be estimated by using the data from this study. According to the data collected over two years, CP intake can be consistently correlated to fifteen-minute grazing intervals, up to forty-five minutes. Mature cattle can obtain approximately 0.45 pound of CP from winter pasture in the initial fifteen-minute grazing interval, which is equivalent to 1.2 pounds (DM) of a 38 percent CP cube or 2.25 pounds (DM) of a 20 percent CP cube. The 0.67 pound of CP intake for the thirty-minute grazing interval represents a DM equivalent of about 1.75 pounds of 38 percent cubes or 3.3 pounds of 20 percent cubes. The 0.85 pound of CP intake for the forty-five-minute grazing interval equals roughly 2.2 pounds (DM) of 38 percent cubes or 4.25 pounds (DM) of 20 percent.
The table summarizes the CP intake on winter pasture and the relative values for 38 percent and 20 percent cubes on a DM and an as-fed basis. It is worth noting that we performed this study by using steers weighing in excess of 1,500 pounds, which is larger than most mature cows. However, the steers in this study were not grazing in direct competition with one another, as would be the situation with a cow herd. The objective of this study was to develop guidelines for limit-grazing winter pasture as CP supplement for mature cattle. We feel these numbers would be comparable to those in a producer's situation as long as forage availability were not limited.
As within any feeding program, there is no substitute for the eyes of the producer. Regardless of the requirements and feeding regimen, adjustments relative to grazing interval and intake may be necessary to fit a specific situation.