USU Irrigated Pasture Grazing Study
USU Irrigated Pasture Grazing Study

Dr. Jennifer MacAdam
USU Forage Plant Physiologist

    A new grazing study has been established at USU�s Caine Dairy Center in Wellsville to test the persistence of eight different grass-legume pasture mixtures under rotational stocking management. When rotationally grazed irrigated pastures are planted with appropriate grass and legume forage species, they can provide several tons of high-quality feed during the growing season. Because the forage is grazed, producers save the time and effort otherwise required to harvest this feed for hay or silage and to spread the manure that would accumulate in dry lots.

    The study will determine the seasonal distribution of yield, forage quality (NDF, ADF, TDN, and crude protein), and botanical composition of eight grass-legume mixtures as affected by grazing. Quality and yield are affected differently each year by climate, and the grass-legume composition of mixtures can change over time as either the grass or the legume increases as a percentage of the total dry matter in the pasture. Therefore, this study is designed to continue for at least three years so results will more accurately predict on-farm forage quality, yield, and grass-legume balance.

    For this study mixtures were planted in 3600 ft2 (1/12th acre) paddocks, and animals will be moved from paddock to paddock each day. Animal numbers will be varied from as few as three to as many as are required to utilize the available forage over the course of one day. In a production situation, the size of the paddock could be varied to provide the needed tonnage of feed, but for this study, paddock size must be held constant. Under rotational stocking management, grazing all the available forage quickly and then removing the animals allows most of the forage to be used when its quality is high. The paddock is then allowed to rest and regrow fully before being grazed again. Grazing eliminates harvesting and storage losses, and rotational stocking management further minimizes forage waste and maximizes forage quality.

    The study is fully replicated four times, and each mixture will also either be grazed early each spring or allowed to mature to the early heading stage and harvested for hay before grazing begins, so the study includes a total of 64 paddocks. Allowing pastures to grow longer in the spring before the first harvest is especially beneficial to the grass component of the mixture, because grasses begin growth earlier than legumes. Harvesting mixed pastures early in the spring tends to favor the legume component by decreasing grass competition when legume growth is just getting underway, and may lead to legume dominance of a mixture. Legume dominance of a grass-legume mixture isn�t always desirable, especially with bloat-causing legumes like white clover or alfalfa.

    Each paddock will be grazed according to its rate of regrowth. As a general rule, taller-growing grasses such as tall fescue and orchardgrass should be allowed to grow to about 10 inches before harvesting, and stubble height after grazing should be about 3 inches. For lower-growing grasses such as perennial ryegrass, grasses should be grazed at about 6 inches, and a stubble of 1-1/2 inches should be left. Stubble height is important because grasses store reserves for regrowth in the stubble or stem base of the shoot rather than in their roots. Some grasses such as smooth brome or timothy have storage organs below the soil surface, but the grasses that are most adapted to rotational grazing do not, so removing too much stubble by close grazing will extend the period needed for regrowth to begin. Putting too much pressure or too high a stocking rate on a pasture will also decrease the overall quality of the forage removed, because animals are less able to be selective. For dairy production, allowing the milking herd to graze first followed by the dry cows and heifers gives the animals with the highest nutritional needs the opportunity to select the highest-quality forage, while the needs of the dry cows and heifers are met by the remaining lower-quality forage.

    Each paddock will be sampled before it is grazed, so four paddocks or one from each replication will be sampled every day. One of the unique challenges of rotational grazing in the Intermountain West is coordinating irrigation and grazing, because the forages used for rotational grazing are shallow-rooted compared with alfalfa, and therefore need to be irrigated twice as often as alfalfa. Pastures are usually irrigated just after grazing, then at least once during regrowth, but the soil must have dried enough to avoid damage to plants before pastures are grazed again. Solid set hand lines for irrigation were installed for the study at the Caine Dairy Center to allow paddocks to be irrigated, sampled, and animals to be moved each day without the additional logistical challenge of moving pipe to the next set of paddocks requiring irrigation. While this is a deviation from the realistic production situation, it should result in a more efficient use of labor during the study.

    In a recently completed clipping or simulated grazing study of 30 grasses and grass-legume mixtures, mixtures on average yielded twice as much as grasses grown alone. The presence of legumes in a pasture not only increases dry matter and crude protein through the contribution of the legume, but the yield of the grass component itself is increased by association with the legume. While a clipping study is the least expensive way to test a large number of mixtures that have potential for high productivity in a particular climate like the Intermountain West, only a grazing study can include all the variables encountered by both grazing animals and forages in pastures, so the eight mixtures shown to have the greatest potential for irrigated pastures in this region were carried forward from the clipping study to the grazing study.

    Averaged over three years, the highest-yielding grasses in the clipping study were tall fescue, meadow brome, and orchardgrass, which produced 4.5, 3.3 and 2.7 tons of dry matter (DM) per acre per year, respectively, when no legume was included. The other grasses were early- and late-maturing varieties of perennial ryegrass and Kentucky bluegrass, and their yields when planted alone were 1.7 (early), 1.5 (late), and 1.1 tons DM per acre per year, respectively. Mixtures with tall fescue, meadow brome, orchardgrass, and perennial ryegrass will be studied under grazing. Although perennial ryegrass is less persistent than the other three grasses, its nutritional quality is outstanding, and it is the most desirable grass to use for grazing-based dairy production.

    The legumes used in the clipping study were alfalfa, birdsfoot trefoil, white clover, and cicer milkvetch. The mixtures containing white clover were consistently the highest-yielding averaging 5.3 tons DM per acre per year. Mixtures with alfalfa averaged 5.1, those with birdsfoot trefoil averaged 5.0, and mixtures with cicer milkvetch were consistently the lowest-yielding, averaging 4.1 tons DM per acre per year. Both birdsfoot trefoil and cicer milkvetch are non-bloating legumes. Based on the results of the clipping study and the compatibility of specific grasses and legumes, birdsfoot trefoil and white clover will be used as the legume components of mixtures under grazing.

    The eight mixtures chosen for the grazing study yielded between 4.5 and 6.0 tons DM per acre per year over three years under simulated grazing. Our goal is to determine the long-term usefulness of these mixtures for grazing-based livestock production in irrigated pastures of the Intermountain West.

    If you have any questions or would like to have more information about this study, please contact Dr. Jennifer MacAdam (phone: (435) 797-2364, e-mail: jenmac@cc.usu.edu. ©