D. Barbour
Analyst · Baird
Thanks, Mike. Good morning, everyone. We appreciate your patience and for joining us on today's call.
Fiscal 2021 started off strong as demand and business activity in the first quarter remain stable at the top with some underlying variability by state and end market. Organic sales grew 3%. And recall that we communicated to you in May that customers bought approximately $20 million of product in the fourth quarter of fiscal 2020 due to pre-buying in our construction end markets and favorable weather conditions in our agricultural end market.
Absent this dynamic, first quarter organic sales would have increased 8% and nonresidential would have been flat year-over-year. Domestically, we had strong performance in key growth states like the Carolinas, Florida, the Southeast and Utah, and we were able to offset sales in states that reduced construction activity due to the COVID pandemic early in the quarter.
As a whole, we benefited from our national presence as well as our geographic and end market exposure, including the increased exposure of the Infiltrator and ADS' focused homebuilder programs provide to the residential end market and our strong presence in the agricultural market.
We had another strong quarter in the domestic agriculture business where sales grew 36%. This growth was primarily driven by actions previously taken to increase our focus on performance in this end market as well as positive underlying demand in this market and what was a strong spring selling season.
A lot of good work is being done in the sales and operations initiatives we defined as part of the renewed focus on agriculture at ADS, and the fall season is shaping up favorably.
International net sales decreased 9% in the quarter. Sales in our Canada business did well, but it was not enough to offset weakness in Mexico in our Exports business. We have recently hired a new Senior Vice President of our International segment, Tom Waun. Tom comes to ADS after a successful career at Emerson, and we are excited to have him as a member of our team. Tom has decades-long experience in executive management, strategy and sales, and I look forward to working with Tom to improve the performance of our International segment.
Infiltrator once again exceeded revenue expectations with sales growth accelerating as we progressed through the first quarter. Infiltrator sales increased across their product portfolio with a strong underlying demand in the residential and repair-remodel end markets. Roy and his team believe, and I agree, that the favorable trends for single-family housing brought on by the pandemic are quite favorable for Infiltrator, so we are making additional capital and resource investments there.
As we get into the second quarter, demand looks very similar to what we experienced in the first quarter. Our order book, project tracking, book-to-bill ratio and backlog are all positive, and we feel good about the first half of our fiscal 2021. We expect the normal seasonal patterns to apply to the business, which only sharpens our focus for this first half of the year.
As mentioned previously, our national presence, distribution model and end market exposure enable us to capitalize on growth and activity across the U.S. As you can see on the chart on the screen, our residential end market exposure has increased to 38% of domestic sales, our second largest domestic end market behind nonresidential. We view this as favorable, given current market trends and the uncertainty in the nonresidential market. The residential market should benefit from single-family housing undersupply and potential future suburban trends as people look to spread out in the aftermath of the COVID pandemic. This dynamic should also benefit horizontal, nonresidential and infrastructure development as developers look to support the increase in single-family homes and communities.
I think in the first quarter with the residential portions of ADS and, of course, Infiltrator growing to double digits, the success of our renewed focus on the agriculture market and the uptick in the sales to the retail segment, driven by the DIY and stay-at-home project activity, demonstrated to us the power of this end market diversity and we've initiated several programs to increase success across these markets so we can be prepared for changes in demand across our business.
From a profitability standpoint, we achieved record adjusted EBITDA in the first quarter. Organic adjusted EBITDA margin increased 830 basis points, driven by favorable material costs, lower manufacturing and transportation costs driven by our operational initiatives, contributions from the proactive cost mitigation steps announced in March and leverage from the growth in Pipe and Allied Products.
Infiltrator also has record profitability in the quarter due to favorable material costs, the contributions from the synergy programs and continued execution of their proven business model. The synergy programs are right on track to achieve the run rate synergies we've previously communicated.
Similar to my comment earlier on the second quarter revenue, the second quarter profitability trends continue in much the same way as the first quarter at both ADS and Infiltrator. We are making good progress on our operational improvement initiatives within our manufacturing and distribution network. Material pricing remains favorable to the prior year, though the comparison will become more difficult as the year progresses.
We will continue to watch our spending very closely as we deal with many of the same issues that other companies are dealing with. Reopening is presenting challenges, including recruiting and retaining production workers, absenteeism, and all of these challenges we have to deal with on a daily basis. We've had roughly 80% of the salaried workforce, including sales, pretty much working from home since late March. So there are a lot of issues you would expect that we're dealing with daily. We have made adjustments and proven to ourselves that we know how to run the business in these conditions, and that's what we'll to focus on as we manage through this period of unique circumstances.
There's no doubt that uncertainties exist for future demand. We will be focused on disciplined execution and doing the basics well as we move forward through fiscal 2021 and build on the strong start at both ADS and Infiltrator.
With that, I'll turn it over to Scott Cottrill to further discuss our financial results.