(CBSDFW/AP) – Four minor league pitchers were suspended this season by Major League Baseball after they were caught using illegal foreign substances to doctor baseballs, signaling a stronger crackdown in the game’s feeder system than in the big leagues during a historically dominant stretch by pitchers.

The suspended minor league players have been in the Texas Rangers, White Sox and San Francisco Giants systems.

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The use of homebrewed tacky substances – mixtures often involve sunscreen and rosin – is suspected to have spiked in recent seasons as pitchers have learned the extent to which illicit sticky stuff can increase the spin rates on their fastballs, making the pitches more effective.

Many believe the proliferation of those grip aids is partially responsible for baseball’s offensive freefall. There were six no-hitters this season, one shy of the record since 1900, and the league’s .236 batting average was the lowest through May 31 since 1968.

MLB has increased oversight in the majors with efforts to inspect more baseballs this season, but the data collection has not led to penalties for players. Suspensions at the big league level are subject to bargaining with the players association.

Minor league players don’t have a union.

MLB confirmed to The Associated Press on Wednesday that four players were suspended.

White Sox right-hander Marcus Every of Low-A Kannapolis was the first, receiving a 10-game ban from MLB after he was ejected from a May 13 game when umpires discovered a foreign substance.

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Three more players were suspended 10 games after being busted last weekend – Sal Biasi of High-A Winston Salem in the White Sox system, Kai-Wei Teng of High-A Eugene with the San Francisco Giants and Mason Englert of Low-A Down East with the Texas Rangers.

Some have called for similar enforcement of existing rules banning substances at the major league level.

Ace Trevor Bauer, now with the Los Angeles Dodgers, strongly and publicly urged MLB to take action on the matter early in his career. He went mum on the subject starting last season, when his own spin rates spiked as he won the NL Cy Young Award with Cincinnati.

Cardinals manager Mike Schildt said last week that sticky substances used by pitchers are “baseball’s dirty little secret” after St. Louis right-hander Giovanny Gallegos was ordered to switch hats when umpires identified a substance on the bill. Schildt believed Gallegos was being unfairly singled out for a widely committed offense.

“Did Gio have some sunscreen at some point in his career to make sure he doesn’t get some kind of melanoma? Possibly,” Schildt said. “Does he use rosin to help out? Possibly. Are these things that baseball really wants to crack down on? No.”

MLB decided before this season to increase its monitoring of baseballs in an attempt to suppress use of foreign substances by pitchers.

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A March memo from MLB executive vice president of baseball operations Mike Hill to teams laid out a three-point plan for cracking down that included game day compliance officers to monitor areas near the field, a system for submitting baseballs that come out of play to the commissioner’s office for inspection and a review of MLB Statcast data to analyze changes in spin rates among pitchers suspected of using foreign substances.