dairy industry conditions and reduced demand from poultry customers. Net sales of vaccines declined $2.4 million, or 7%, due to turbulent economic conditions in certain international countries and the loss of a domestic distribution arrangement; volume growth in other international markets partially offset the reductions.
Mineral Nutrition
Net sales of $117.2 million for the six months ended December 31, 2018, increased $5.5 million, or 5%. Higher average selling prices were the primary driver of the increased revenue, while volumes were approximately level with the prior year. Our selling prices of mineral nutrition products generally move in direct correlation with the underlying commodity costs.
Performance Products
Net sales of $30.5 million for the six months ended December 31, 2018, increased $4.5 million, or 18%, due to volume growth of copper-based products, coupled with higher average selling prices of certain industrial chemical products.
Gross profit
Gross profit of $134.4 million for the six months ended December 31, 2018, increased $4.1 million, or 3%, as compared to the six months ended December 31, 2017. Gross profit decreased to 32.1% of net sales for the six months ended December 31, 2018, as compared to 32.6% for the six months ended December 31, 2017. Gross profit for the six months ended December 31, 2017 included $1.7 million of acquisition-related cost of goods sold.
Animal Health gross profit increased $3.8 million due to volume growth and favorable unit costs and product mix in MFAs and other, partially offset by volume declines in both the nutritional specialty and vaccine categories and increased vaccine manufacturing costs. Mineral Nutrition gross profit decreased $2.7 million, primarily due to unfavorable product mix and constrained pricing in a competitive environment. Performance Products gross profit increased $1.4 million, primarily due to volume growth. Gross profit increased $1.7 million due to acquisition-related cost of goods sold included in the three months ended December 31, 2017.
Selling, general and administrative expenses
Selling, general and administrative expenses (“SG&A”) of $85.9 million for the six months ended December 31, 2018, increased $1.9 million, or 2%, as compared to the six months ended December 31, 2017. SG&A for the six months ended December 31, 2018, included $1.1 million of stock-based compensation and a $1.5 million benefit from the cancellation of a certain business arrangement. SG&A for the six months ended December 31, 2017, included $0.4 million in acquisition-related transaction costs and $0.9 million in acquisition-related compensation costs. Excluding the effects of these costs, SG&A increased $3.6 million, or 4%.
Animal Health SG&A increased $1.3 million, driven by sales force expansion and increased investments in marketing and product development. Mineral Nutrition and Performance Products SG&A were each little changed from the prior year. Corporate costs increased $2.6 million due to higher business development expenses and professional fees. The stock-based compensation, cancellation of the business arrangement, acquisition-related transaction costs and acquisition-related compensation costs resulted in a $1.7 million reduction in SG&A.
Interest expense, net
Interest expense, net of $5.8 million for the six months ended December 31, 2018, decreased $0.4 million, or 6%, as compared to the six months ended December 31, 2017, primarily due to increased interest earnings on short-term investments.
Foreign currency (gains) losses, net
Foreign currency (gains) losses, net were insignificant for the six-month periods ended December 31, 2018 and 2017.