Weed Encroachment in Established Alfalfa as Affected by Cutting Frequency

Crazing-tolerant alfalfa (Medicago sativa L.) cultivars are more persistent than hay-type cultivars under severe grazing pressure but the influence of defoliation on weed encroachment is not well defined. Our objective was to determine the effect of rest interval between cuttings on encroachment of large crabgrass [Digitaria sanguinatis (L.) Scop.1 or common bermudagrass [Cynodon dactylon (L.) Pers.] into a grazing-tolerant alfalfa cultivar. We used small-plot field experiments with four replications of ‘Alfagraze’ alfalfa to evaluate nine treatment combinations of (i) harvesting at 2-, 4-, and 6-wk intervals, and (ii) overseeding with common bermudagrass, large crabgrass, or nothing. Experiments were conducted for 3 yr at Eatonton, CA, on Davidson loam (clayey, kaolinitic, thermic, Humic Paleudult) and at Athens, CA, on Cecil sandy clay loam (clayey, thermic, kaolinitic, %pic Hapudult). Alfalfa yields were similar at both locations when harvested at 4- or 6-wk intervals. Crass invasion was virtually nil with the 4- and 6-wk treatments at Eatonton but at Athens there was some grass encroachment, probably a result of soil compaction,which adversely affected overall alfalfa growth. Cutting at 2-wk intervals reduced alfalfa yields to 50% of the 4- or 6-wk cut treatments and resulted in heavy bermudagrass encroachment during the third year. Alfalfa cut every 4 or 6 wk persisted well but stands were reduced when cut every 2 wk Results suggest that frequent defoliation will not only reduce alfalfa yield but also favors grass encroachment. Thus, defoliation methods that keep alfalfa canopies at a high level would appear to be desirable in resisting bermudagrass and crabgrass invasion and increasing yield of grazed alfalfa. Research QuestionRecommended management for alfalfa pasture is rotational stocking to graze off the forage in a few days followed by a 5 to 6 wk rest period. Unfortunately, in actual farm practice, this recommendation is often modified with shorter rest periods or continuous stocking, which may result in grass invasion and alfalfa stand loss. The present study evaluated the effect of rest interval between cutting on encroachment of large crabgrass and common bermudagrass in a sward of a grazing-tolerant alfalfa cultivar at two locations in north and central Georgia. Literature SummaryAlfagraze is much more tolerant of continuous stocking at high grazing pressure than hay-type alfalfa cultivars. However, under these conditions in the southeastern USA, invasion of the sward by large crabgrass and common bermudagrass is a serious problem. In contrast, under a hay-cut system, rapid regrowth by alfalfa provides the sward with enough shade to reduce grass establishment competition. Information is lacking in the southeastern USA on the effect of rest interval between cuttings or grazings on encroachment of warm season grass weeds in alfalfa. Study DescriptionAlfalfa yield, persistence, and weed invasion were determined for Alfagraze alfalfa grown alone or overseeded with large crabgrass or bermudagrass and harvested at 2-, 4-, and 6-wk intervals (HI) over 3 yr at Athens and Eatonton in the Georgia Piedmont. Applied QuestionsHow did harvest interval affect alfalfa yield? Alfalfa yields were similar at 4- and 6-wk HI. Generally, hay-type alfalfa cultivars are more productive when harvested at 6 than 4-wk HI but the similar yields of Alfagraze at both HI indicate its greater tolerance to frequent defoliation. However, cutting at 2-wk HI reduced alfalfa yield to 50% ofthat at 4- to 6-wk HI. Alfalfa stands were also sharply reduced by 2-wk HI, with loss being greater when grass invasion occurred. How did harvest intervals affect grass encroachment? Grass invasion was not a problem when harvested at 4 and 6 wk at Eatonton but at Athens there was some grass encroachment by the third year, a result of compacted subsoil, which adversely affected alfalfa growth. Cutting at 2 wk resulted in heavy bermudagrass encroachment, and to a lesser extent crabgrass, by the second and third years. Grass invasion at the 2-wk HI reduced alfalfa stands by the third year to less than 50% of grass-free alfalfa. Grass encroachment was eliminated or minimal when a vigorous stand of Alfagraze alfalfa was cut at 4-wk HI, suggesting that rotational stocking at a similar HI should resist grass invasion in a pasture. RecommendationExcellent forage production and resistance to grass invasion when Alfagraze alfalfa was harvested at 4-wk HI suggests the importance of maintaining an adequate rest interval between grazings. Although most pasture areas in the southeastern USA have abundant seed banks of bennudagrass and large crabgrass, which will soon dominate continuously stocked or 2-wk rotationally stocked alfalfa, maintaining a 4-wk rest period between short grazing periods should be effective in slowing grass encroachment. Use of a grass herbicide is another option for controlling weeds in alfalfa.