Features Agronomy Corn
Power hilling timing affects yield and quality

Traditionally, commercially grown potatoes are hilled in the production cycle
between emergence and canopy close. Hilling improves drainage, minimizes tuber
greening, minimizes frost damage, aids in weed control and facilitates harvesting.
Cultivation may benefit potatoes by aerating and improving the soil structure,
but it may be detrimental to potato growth if soil structure is disturbed, potato
roots are pruned, or foliage is damaged.

"What we found when we purchased a power hiller for our research was that
there really were not any solid recommendations for our area. We talked to custom
applicators and some went at ground crack, while others did it as soon as they
planted and others went a bit later," says Michele Konschuh, a potato research
scientist with the Crop Diversification Centre South division of Alberta Agriculture,
Food and Rural Development at Brooks. "As more growers move to power hilling,
we thought it would be good to have a better understanding of the impacts on
potato development, yield and quality."

To help determine the best time for power hilling, Konschuh set up a three
year research trial in 2003 to compare power hilling to conventional disc hilling.
Conventional hilling is typically conducted when plants are approximately 30cm
tall because there is little risk of covering the foliage. Vines of larger plants
may, however, sustain greater damage from hilling than smaller plants, and the
possibility of damaging roots and stolons increases as the plants increase in
size. General recommendations for post-emergence hilling with conventional equipment
is for it to be completed before the plants are 20cm in height to avoid damage
to roots and foliage, and power hilling should be completed prior to emergence
to avoid covering the plants.

The research project on Russet Burbank potato compared power hilling at five
different times, including immediately after planting, at ground crack, at emergence,
stolon hooking and bud formation (plants five to 12 inches tall). Disc hilling
was done at emergence, stolon hooking, buds forming and row closure stages.

Daily minimum and maximum temperature in the hills was monitored throughout
the season in 2004. Tuber yield was measured, graded and marketable yield determined.
Specific gravity was determined, and brown centre, hollow heart and other internal
defects were assessed.

Too early, too late or just right
The weather conditions over the three years were extremely variable, providing
Konschuh with an opportunity to compare weather effects on hilling as well:
2003 was hotter than normal, 2004 was cooler than normal, and 2005 was cool
and wet with heavy rainfall throughout June. She found that the weather had
a large impact on the temperature within the hills and, as a result, influenced
potato development in May and June.

"We found that with power hilling, you could go too early. If you went
immediately after planting, it insulated the potato and in cool years, that
slowed down potato development," explains Konschuh.

In general, Konschuh found that climate had a greater impact on soil temperatures
in potato hills than the method or timing of hilling. Timing of hilling and
method of hilling had less impact on minimum soil temperatures in the spring
than on maximum soil temperatures. Maximum soil temperatures differed by as
much as four degrees C between treatments.

Power hilling immediately after planting appeared to prevent the hills from
warming up as much as the control treatments. The cooler temperatures in these
hills may have delayed emergence and may account for lower yields relative to
the check or to hilling at ground crack or emergence. In general, power hilling
treatments had lower maximum soil temperatures and higher minimum soil temperatures
in the spring compared to disc hilled treatments. This was beneficial in hot
springs, but negatively affected potato growth in cool springs.

In the fall, power hilling provided better insulation from the cold nights.
"Insulation helps with bulking in the fall, because it keeps the soil temperature
higher. Power hilling can help keep minimum temperatures higher," explains
Konschuh.

Power hill at ground crack to emergence for highest
returns
Power hilling at ground crack (three weeks after planting) resulted in the highest
gross yield, the highest marketable yield and good specific gravity and fry
colour relative to other treatments. Gross yield and marketable yield were reduced
when potatoes were power hilled after tuber initiation (six and seven weeks
after planting). The optimum time to hill with a power hiller appears to be
before plant emergence, although power hilling up to stolon hooking still resulted
in acceptable marketable yields.

Power hilling immediately after planting appeared to delay emergence and reduce
yield, possibly because soil temperatures remained cool longer. Late hilling
caused a reduction in total yield, however, late power hilling was more detrimental
to marketable yield than late disc hilling. If power hilling has not been completed
by the time plants are two to five inches tall, disc hilling may be better than
power hilling.

"Sometimes weather doesn't allow power hilling at ground crack, or growers
can't cover all their acreage at the optimum time. If they get delayed, there
may be an impact from power hilling too late, and it might be better to disc
hill," says Konschuh. "The window of opportunity for disc hilling
is wider than for power hilling."

Power hilling at emergence resulted in the highest specific gravity, however,
few significant differences in specific gravity were observed between treatments.
The best fry quality was observed for treatments hilled at emergence. Fry colours
were very good for all treatments, regardless of timing or method of hilling.

The greatest uniformity of tuber size was observed when Russet Burbank potatoes
were power hilled at emergence. Poor timing of power hilling decreased the uniformity
of tuber size.

In 2004, power hilling Russet Burbank potatoes at ground crack or at emergence
resulted in the best combination of marketable yield, good fry colour and uniform
tuber size. Konschuh is still crunching the numbers for 2005, but says the same
general trends exist. Power hilling at ground crack to emergence provides the
best marketable yield.

"If we could predict the weather, it would be easier to decide on the
best time for power hilling," says Konschuh. "If you go early and
the weather turns cool, it is like keeping the potatoes in a refrigerator. If
wet and cold, you'll have less yield than if you go two weeks later. More commonly,
though, in southern Alberta we get hotter weather, so power hilling at ground
crack to emergence is typically the best recommendation." -30-