A combination of good, well-timed weed control and soil fertility management, as well as grazing/cutting at appropriate times, are important to keep your crop of lucerne healthy.
In mature crops, for example those over 12 months old, weed control is mostly done during winter when the crop is dormant to reduce damage. A spray that removes old leaves that could be harbouring aphids and disease mixed with residual herbicide lays down a barrier preventing weeds from germinating in the spring.
Walking the paddocks during late autumn and identifying weeds and their numbers helps this decision. Then, during mid-winter, apply a mixture of a contact burn down spray (containing paraquat) and another residual herbicide (usually containing atrazine), thus browning down weeds and leaving a residual herbicide layer on the soil surface killing any newly emerging ones.