Herbage Production, Weed Occurrence, and Economic Risk from Dairy Manure Applications to Alfalfa

The spreading of dairy manure on forage legumes requires more information about the impact of manuring on yield, stand persistence, and weed incidence. The research objective was to evaluate the effect of dairy manure applications on herbage yield and weed infestation of established alfalfa (Medicago sativa L.) and to assess the economic risks of this management. Field experiments were initiated in 1990 on an Occum fine sandy loam variant soil (coarse‐loamy, mixed, mesic Fluventic Dystrochrept) at Deerfield and on an Agawam fine sandy loam soil (coarse‐loamy, mixed, mesic Typic Dystrochrept) at Sunderland in western Massachusetts. Treatments consisted of no N, low and high manure‐N (100 and 300 lb N/acre from approximately 5000 and 15 000 gal liquid manure/acre per year, respectively), and low and high N fertilizer (100 and 300 lb N/acre per year from ammonium nitrate, respectively). Liquid dairy manure or ammonium nitrate was spread on the surface immediately after the first cutting in June 1990 and again in June 1991. Application of manure or ammonium nitrate at low or high rates to alfalfa increased total herbage yield compared with no N treatment at the Sunderland site but no response was observed at the Deerfield site over the 2 yr of study. Application of manure or ammonium nitrate to alfalfa at both the sites had no significant effect on overall weed incidence compared with unfertilized alfalfa. Manure applications to alfalfa followed by a dry period can influence weed growth more than in a normal year as it was noticed in the August 1991 cutting. The differences in alfalfa yield due to manuring and no N application were not of economic significance. This alternative practice of manure application to alfalfa does not involve a significant economic risk for dairy farmer. The results show that dairy manure can be applied to alfalfa without any adverse effects on herbage yield or weed incidence and with no economic risk on productivity. This alternative management strategy for surplus dairy manure can effectively reduce overapplications of manure on corn (Zea mays L.) fields and increase the total land area available for spreading. Research Question Dairy manure management has become an important concern due to its potential for nitrate pollution. Alternative manure management practices, such as application of manure on alfalfa, have become an important option for dairy farmers. However, many dairy farmers are reluctant to spread manure on established alfalfa as they are concerned with affecting stand longevity and purity. The research objective of this study was to evaluate the effect of dairy manure applications on herbage yield and weed incidence of established alfalfa and to provide an economic assessment of this management. Literature Summary Several researchers have reported the efficiency of alfalfa in using soil N. Nitrogen fertilizer often is applied to alfalfa in the establishment year with varying degrees of yield response. Minnesota research showed that application of manure had a stimulating effect on weed growth during an establishment year, but weeds were not a problem after the first cutting. However, other studies have shown competition between weeds and alfalfa, especially in declining alfalfa stands. Nevertheless, since dairy manure applications on alfalfa can potentially increase the total area available for manure spreading and decrease manure application to corn, there is need for both an environmental and economic risk evaluation of this practice. Study Description Experiments were initiated in June 1990 on a 1‐yr‐old stand of alfalfa in Deerfield and a 2‐yr‐old stand of alfalfa in Sunderland in western Massachusetts. The soil at the Deerfield site was an Occum fine sandy loam and the Sunderland site was an Agawam fine sandy loam. Treatments consisted of a control (zero N), low and high manure rates (100 and 300 lb N/acre per year from approximately 5000 and 15 000 gal liquid manure/acre, respectively), low and high N fertilizer (100 and 300 lb N/acre per year from ammonium nitrate, respectively). Liquid dairy manure was applied to alfalfa by surface spreading immediately after the first cutting. Alfalfa was harvested three times a year, and total herbage yield was recorded. After the harvest of each alfalfa cutting, random subsamples were collected and separated into alfalfa and broadleaf and grass weeds. Percentage of total weeds in the herbage was calculated on dry weight basis. The economic risks involved with this alternative manure management were assessed by the certainty equivalent concept, which depends on both the relative frequencies of crop yields and a farmer's degree of aversion to risk. Applied Questions Does liquid dairy manure application to alfalfa affect herbage yield? Application of manure at the low (100 lb N/acre) or the high rate (300 lb N/acre) to the 2‐yr‐old stand of alfalfa increased total herbage yield at the Sunderland site compared with no N treatment but no significant Full scientific article from which this summary was written begins on page 495 of this issue. response was noticed on the 1‐yr‐old stand of alfalfa at the Deerfield site over the 2 yr of study (Table ). Herbage yields at both the sites in 1991–1992 were reduced by dry weather. Environmental consideration for a potential pollution of ground and surface waters should be an important aspect of deciding the rates of manure that can be spread on alfalfa. How do manure applications affect weed population in alfalfa stands? The 2‐yr experimental results showed that the application of manure at the low or the high rate to alfalfa at both sites had no significant effect on weed incidence compared with alfalfa receiving no N. The percentage of weeds in alfalfa stands increased significantly in all the treatments at the Sunderland site during 1991–1992. This rise in weed incidence is probably due to combined effects on an aging alfalfa stand (4‐yr‐old) and increased competition between weeds and alfalfa for soil moisture in a drought year. The effects of applying manure to alfalfa on herbage yield or weed incidence can vary depending upon the age of the alfalfa stand and climatic conditions. Manure applications to alfalfa followed by a dry period, can influence weed growth more than in a normal year. Is there any economic risk involved in applying manure to alfalfa? The differences in alfalfa herbage yield with conventional production practices vs. manure application to alfalfa would be regarded as negligible by most farmers and are not of economic significance. Hence, manure can be applied to land planted to alfalfa without any economic risk and this strategy could prevent excess manure applications on other fields. Total herbage yield as affected by application of manure or ammonium nitrate at two locations. Dearfield site Sunderland site Treatments 1990–1991 1991–1992 Mean 1990–1991 1991–1992 Mean ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ tons/acre per year ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ No N 6.3 5.9 6.05 6.1 4.3 5.22 Low manure 6.4 5.9 6.12 6.3 4.8 5.55 High manure 6.6 5.1 5.85 6.3 4.8 5.56 Low ammonium nitrate 6.6 6.0 6.28 6.6 4.5 5.56 High ammonium nitrate 6.6 5.9 6.24 6.7 4.9 5.80 LSD at 0.05 NS 0.4 NS NS 0.3 0.28 Mean 6.5 5.8 6.11 6.4 4.7 5.54