(Bloomberg) -- After trailing gold last month, silver is outshining the yellow metal again.

An ounce of gold bought as little as 73.1 ounces of silver on Friday, the smallest ratio in almost a month. Holdings in ETFs backed by silver are nearing an all-time high. This month, spot silver is up about 8.5%, compared with gold’s 5% increase.

Gold and silver have rallied this year on speculation that U.S. interest rates will stay lower for longer. Low rates are a boon to precious metals, which offer returns only through price gains. Silver is getting an added boost from supply concerns. Output from silver mines will fall for the first time since 2011, while demand for the metal in uses including industrial products will rise a fourth straight year, CPM Group forecast in April.

"When silver sells off, it sells off faster than gold, but when it rallies, it rallies so much more," Phil Streible, a senior market strategist at RJO Futures in Chicago, said by telephone. "The physical demand for silver is quite high. And if there’s a slump in production, we might see some short squeezes come into play."

Silver for immediate delivery gained 0.4% to $17.341 an ounce, heading for a third straight gain, according to Bloomberg generic pricing as of 9:39 a.m. in New York. Gold for immediate delivery rose 0.3% to $1,273.84 an ounce, also set for a third straight increase.

Traders see a zero percent chance that the Federal Reserve will raise borrowing costs at a meeting next week, compared with 74.5% at the start of the year, according to Fed-funds futures.

Investors added 1,224 tons into silver-backed ETFs this year to 20,080.9 tons. That’s just about 101 tons below the record in 2014. Assets in gold-backed ETFs have climbed by 405.8 tons this year, according to data compiled by Bloomberg.

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