13. REVENUE

We primarily offer the First Defense^® product line to dairy and beef producers to prevent scours in newborn calves. Generally, our products are promoted to veterinarians and dairy and beef producers by our sales team and then sold through distributors. Our primary market is North America. We do sell into select international regions and may expand this international reach in the future. There were no material changes between the allocation and timing of revenue recognition during the year ended December 31, 2018 (under ASC 606) and the year ended December 31, 2017 (under ASC 605). We do not have any contract assets such as contracts for which we have satisfied the performance obligations but do not yet have the right to bill for or contract liabilities such as customer advances. All trade receivables on our balance sheet are from contracts with customers. We incur no material costs to obtain contracts. As of March 31, 2018, we had a backlog of orders (representing purchase orders received from customers which were not fulfilled or paid) worth approximately $1,245,000 for the First Defense^® product line. Before June 30, 2018 we cleared all of this backlog that was related to orders for our bivalent formats of the First Defense^® product line (which have been re-branded as Dual-Force™ First Defense^®). Demand for Tri-Shield First Defense^® continues to exceed our available inventory, but we are not accepting orders in excess of available inventory, which requires a careful allocation of inventory as it becomes available to address the needs of specific customers. As of December 31, 2018, we had received orders representing a backlog worth approximately $393,000. We had sufficient inventory on hand to satisfy these orders, but this product did not ship to customers until the beginning of 2019 for several reasons including the following: i) most of these orders were received during the last two days of the year, ii) product requiring cold shipment does not ship over a weekend or holiday and iii) normal shipping services were interrupted due to the holiday schedule.

The following table presents our product sales disaggregated by geographic area:

	During the Years Ended December 31,
	2018		2017
United States	$	9,559,142	$	8,626,517
Other		1,427,155		1,804,574
Total product sales	$	10,986,297	$	10,431,091

The following table presents our product sales disaggregated by major product category:

	During the Years Ended December 31,
	2018		2017
First Defense^® product line	$	10,663,265	$	9,814,501
Other animal health		298,932		423,790
Other		24,100		192,800
Total product sales	$	10,986,297	$	10,431,091