Yield data on narrow-row silage corn looks encouraging - especially in the North, points out Bill Mahanna, Pioneer Hi-Bred International's forage product manager.

Mahanna says more uniform plant spacing improves light interception. Also, nutrient uptake and weed control are enhanced.

But the lower silage digestibility that comes with higher plant populations must be considered, he adds.

"Whether you can benefit from narrow rows and higher plant populations for your silage corn depends on current milk prices, soil fertility and texture, your planting and harvesting equipment, hybrid selection and the specific nutritional needs of your cows," Mahanna states.

He points to a recent Pioneer field study in western New York. The same hybrid was planted in 15" rows at four locations and at harvest populations of 28,000 to 55,000 plants/acre.

The results showed a trend for increased silage yields at higher populations, but digestibility dropped. Similar findings have also been reported by Penn State and Cornell University researchers.

Kurt Ruppel, Pioneer dairy specialist in Greenwich, NY, reports that, for every 6,000-plant increase in harvest population, yield increased 1.3 tons. But silage digestibility decreased by 0.74 percentage unit.

Ruppel says that the reductions in digestibility at higher plant populations were due to reduced sugar content, not lower fiber digestibility.

Pioneer's yield increases with higher populations were much greater than those reported from Cornell, but digestibility reductions were almost identical in the two studies.

Based on the Pioneer study results, Ruppel concludes that, in terms of potential milk per acre, 40,000 plants/acre is the optimum population for New York. Research in Wisconsin has shown that populations in the 30,000-33,000 range are optimal for both grain and silage in that state.

"The increased return from added yield must be compared against the decrease in potential milk return from lowered digestibility," says Mahanna.

Milk price is an important factor. At $12/cwt milk, dairy farmers should not plant higher populations if their yields are already in the range of 25 tons/acre (30% dry matter), says Mahanna. As the price of milk increases to, say, $14/cwt, populations that deliver yields in the 22 tons/acre range probably are optimal considering the higher value of the added digestibility in the lower populations.

Here's another option: Plant narrow rows, but leave your plant population the same. The Cornell study showed significant yield gains from 15" vs. 30" rows planted at the same population. For example, at 37,000 plants per acre, 15" rows outyielded the wider ones by 1.6 tons/acre in two out of three years.