Wheel traffic from tractors and harvesting equipment can have an effect on the productivity of alfalfa. Wisconsin data suggests as much as 20% loss in yield over the season when traffic was applied 5 days after harvest. We evaluated traffic effects over the past 4 years in conjunction with the alfalfa performance testing at Fargo. FA-04, a 2004 seeding, had two replicates; FA-05, a 2005 seeding, had three replicates; and FA-06, seeded in 2006, had three replicates treated with or without traffic (one pass with a medium-size tractor) 5 days after harvest following the first, second, and third harvests. No traffic was applied to the first harvest. FA-04 had 8, FA-05 had 12, and FA-06 had 8 varieties/experimental lines included.
Forage yield with traffic was 1.0 tons/acre less in 2005, a year with 7.5 inches above-normal rainfall (Table 1). This is a reduction in yield of 11.7% with traffic. The interaction between cultivar and traffic treatment was non significant for the four harvest and total yield.
Forage yield with traffic was 0.40 and 0.38 tons/acre less in 2006 from FA-04 and FA-05, respectively (Table 2). This amounts to a 9.3 to 9.4% reduction in yield. The 2006 forage-growing year (October 2006 to September 2006) was near normal in rainfall, but the growing season (April to September) was very dry reducing the second- and third-harvest yields. The interaction between cultivar and traffic treatment was non significant in both experiments.
Forage yield with traffic was 0.59, 0.38, and 0.42 tons/acre less than non traffic treatments in 2007 for FA-04, FA-05, and FA-06, respectively (Table 3). This represents a reduction in yield of 9.2, 9.3, and 9.4%, respectively. The interaction between cultivar and traffic treatment was non significant in all three experiments, which was similar to 2006.
Forage yield with traffic was 0.21 and 0.36 tons/acre less than non-trafficked treatments in 2008 for FA-05 and FA-06, respectively (Table 4). This represents only a 2.6 and 4.7% yield reduction, respectively. Traffic effects in 2008 were much less than previous years (averaging about 9 to 9.5% previously). The reason is unclear, but it may have been involved with the cool, wet growing season. The variety by traffic interaction was significant for the fourth harvest of FA-05 while it was non significant in other cuttings and in FA-06 like previous years.
These data to date indicate that traffic has a significant affect on the forage yield of alfalfa averaging about 5 to 9% yield reduction (Table 5). The traffic treatments have been harvested on the same date, which might be causing a greater yield reduction than normally experienced in producer fields. Traffic causes the crushing of regrowth shoots and delays maturity somewhat. If the traffic treatments were harvested at the same maturity, forage yields would be more similar among traffic treatments. In addition, treatments with traffic had the entire plot treated whereas producing fields have only a portion of the field treated with traffic. The soil at Fargo is a Fargo clay, which is a shrink/swell soil; therefore, soil compaction is not a factor. Areas that do not have the smectite soils may have greater reduction in alfalfa yield with traffic.