Am I just sacrificing my spring pasture growth by growing extra grass in winter?

No, the extra dry matter produced by ProGibb in winter is additional to the spring growth that will come with the warmer temperatures. There has been a suggestion that pasture treated with ProGibb in winter may have a slight ‘bounce down’ effect in Spring, however, the effect has be very hard to quantify and is overshadowed by the large jump in production that occurs in Spring.