OTTAWA -- Frustrated farmers are calling for a swift end to a rare standoff between the Senate and House of Commons over a bill that would help ensure rail shipments of grain don't get backlogged again.

They're putting pressure on both MPs and senators to find a quick resolution that will allow Bill C-49, which has been bouncing back and forth between the two parliamentary chambers, to finally become law.

The bill was passed by the House of Commons in November but the Senate last month sent it back to the elected chamber with 18 amendments.

The Trudeau government accepted two of those amendments and modified three others, but rejected the rest.

Rather than defer to the will of the elected chamber as is usually the case, senators voted 43-39 on Wednesday to insist on two further amendments.

They sent the bill back to the Commons on Thursday for a second time -- a rare move that was last employed in 2006, according to the library of Parliament.

The government is now set to move a motion Friday morning to send C-49 back to the Senate with a message rejecting the two additional amendments.

Senators will have to decide at that point whether they will continue to insist on the changes or acquiesce to the will of the Commons.

Conservative senators voted as a block on Wednesday in support of sending the bill back to the Commons for the second time, with support from 11 Independent, non-aligned and independent Liberal senators.

Whether they're prepared to stand firm on the two amendments and bounce the bill back a third time seems doubtful.

Conservative Senate leader Larry Smith said in an interview that he recognizes it is "our responsibility to defer at a certain point in time" to the elected chamber. If the government rejects the amendments a second time, he said deferring to its will "has to be taken into serious consideration."

In the meantime, groups representing grain farmers are urging the two parliamentary chambers to sort out their differences and get the bill passed.

"While not perfect, Bill C-49 as approved by the House of Commons on May 3 has the right combination of tools to improve Canada's grain shipping systems so that farmers can access a more reliable, efficient and competitive railway network," said Canadian Federation of Agriculture president Ron Bonnett in an email to parliamentarians.

"It's late in the game for ongoing changes. After many rounds of review, we need the bill passed so the new law is in place by Aug. 1, when crop shipping season begins. We ask once again: Please pass Bill C-49 ASAP."

Grain Growers of Canada president Jeff Nielsen said his organization is pleased with the amendments the government has already accepted, but disappointed with the additional delay.

"It's just really frustrating," he said in an interview, noting that farmers are already getting orders for this year's grain crop and need to know they'll be able to ship it reliably.

The Alberta Wheat and Barley Commissions are urging the government to accept the Senate's amendments.

"We urge the House of Commons to prioritize passing the bill as amended in a timely manner," Kevin Bender, Alberta Wheat Commission Chair, said in an email statement.

"Bill C-49 offers long-term, competitive solutions that will ensure farmers are no longer at the mercy of an unaccountable freight rail system."

The Canadian Federation of Agriculture estimates that grain farmers have lost billions of dollars over the past few years due to backlogged rail shipments, which has cost them sales and put many in a cash-flow crunch.

Among other things, C-49 would financially penalize railways that fail to deliver promised rail cars on time. It would also require railways to publicly report each summer on their ability to move that year's grain crop, and to publish by Oct. 1 each year a winter contingency plan for keeping shipments moving regardless of bad weather.

The same bill would also create a new air passenger bill of rights, relax international ownership restrictions on Canadian airlines and require railways to install voice and video recorders in locomotives.