Traffic Effects On Forage Yield of Alfalfa.

      Wheel traffic from tractors and harvesting equipment can have significant effects on the productivity of alfalfa (Medicago sativa L.).  Previous research by Dan Undersander and others at the University of Wisconsin suggested that alfalfa forage yield is reduced by wheel traffic 5 days after harvest by 20 to 25% with greater reduction possible when traffic occurred at a later date.  We evaluated traffic effects over the past 4 years in conjunction with alfalfa performance testing at Fargo, North Dakota.  The 2004, 2005, and 2006 performance tests had half the replicates treated with a single pass of a medium-size tractor with duals 5 days after harvest or equivalent to baling the hay 5 days after cutting.  There were 8 entries in 2004 and 2006 seedings and 12 entries in the 2005 seeding.        Traffic reduced forage yields from 2.6 to 11.7% when averaged across the entries during 2005 to 2008 (Table 1).  Forage yield was reduced by traffic 11.7% in 2005 and only 2.6 to 4.7% in 2008.  The difference among years is unclear, but 2008 had some soil water stress.   Dry soils may reduce traffic effects or soil texture (a high clay soil with 6 to 7% organic matter at Fargo) may play a role.  Fargo clay soils are shrink-swell soils (smectite types) or ones that expand and contract during water uptake or drying out.  This reduces soil compaction but would have no effect on bud or young shoot growth, a major cause of the delayed growth often noted with traffic.  This experiment overstates traffic effects since the entire plot received the traffic treatment whereas production fields are not fully impacted by traffic.  Only once in 24 experiment/year/harvest observations was the traffic treatment by variety interaction significant even when AmeriStand 403T was included as a traffic-tolerant check.   We concluded that traffic effects on forage yield are more impacted by the environment and soil type than the cultivar used.   

Table 1.  Traffic effects on alfalfa production at Fargo, ND.
Production	No			%
year	traffic	Traffic	Significance	decrease
	----------Mg/ha----------
	2004 seeding, 8 entries
First, 2005	19.1	16.9	*	11.7
Second, 2006	13.2	12.3	NS	6.8
Third, 2007	17.1	15.8	NS	9.2
Mean	16.5	15.0		9.0
	2005 seeding,12entries
First, 2006	13.4	12.6	10%	6.3
Second, 2007	15.7	14.6	*	7.0
Third, 2008	18.3	17.8	*	2.6
Mean	15.8	15.0		5.1
	2006 seeding, 8 entries
First, 2007	14.7	13.8	*	6.4
Second, 2008	17.1	16.3	10%	4.7