higher revenues of our companion animal product. Net sales of vaccines increased $7.8 million, or 22%, due to increased domestic and international volumes.
Mineral Nutrition
Net sales of $121.1 million for the six months ended December 31, 2021, increased $15.5 million, or 15%. The increase was mainly attributable to an increase in average selling prices, partially offset by a decrease in volume. The increase in average selling prices is correlated with the movement of the underlying raw material costs.
Performance Products
Net sales of $34.4 million for the six months ended December 31, 2021, increased $3.2 million, or 10%, driven by higher volume and higher average selling prices of copper-based products, partially offset by lower sales of personal care product ingredients.
Gross profit
Gross profit of $135.4 million for the six months ended December 31, 2021, increased $3.0 million, or 2%, as compared to the six months ended December 31, 2020. Gross margin decreased 270 basis points to 30.3% of net sales for the six months ended December 31, 2021, as compared to 33.0% for the six months ended December 31, 2020.
Animal Health gross profit increased $0.7 million as increases in product demand and average selling prices were largely offset by higher raw material and logistics costs and unfavorable product mix. Mineral Nutrition gross profit increased $3.1 million, with increases in average selling prices partially offset by increased raw material costs. Performance Products gross profit decreased $0.8 million due to higher raw material costs and production costs and unfavorable product mix.
Selling, general and administrative expenses
SG&A of $98.4 million for the six months ended December 31, 2021, increased $1.6 million, or 2%, as compared to the six months ended December 31, 2020. SG&A for the six months ended December 31, 2021, included a $1.2 million gain on sale of investment. SG&A for the six months ended December 31, 2020, included $1.1 million of stock-based compensation. Excluding these items, SG&A increased $4.0 million, or 4%.
Animal Health SG&A increased $2.7 million primarily due to an increase in headcount and employee-related costs. Mineral Nutrition and Performance Products SG&A were comparable to the prior year. Excluding the gain on sale of investment and stock-based compensation, Corporate expenses increased $1.1 million due to strategic investments.
Interest expense, net
Interest expense, net of $5.8 million for the six months ended December 31, 2021, decreased $0.2 million, or 3%, as compared to the six months ended December 31, 2020. Interest expense decreased due to favorable borrowing rates, partially offset by higher levels of debt outstanding and lower interest income from short-term investments.
Foreign currency (gains) losses, net
Foreign currency gains, net for the six months ended December 31, 2021, amounted to net gains of $2.1 million, as compared to net gains of $3.0 million for the six months ended December 31, 2020. Foreign currency (gains) losses, net primarily arose from intercompany balances. Current period gains were driven by the movement of the Turkish and Brazilian currencies relative to the U.S. dollar.
Provision for income taxes
The provision for income taxes was $9.1 million and $7.5 million for the six months ended December 31, 2021 and 2020, respectively. The effective income tax rate was 27.6% and 22.9% for the six months ended December 31, 2021 and 2020, respectively. The provision for income taxes for the six months ended December 31, 2021, included a $0.5 million expense from changes in