A sensor assembly comprising a plant matter sensor mounted in a support frame, wherein the sensor includes: a pair of parallel spaced apart substantially vertically disposed sensor arms, a first of said arms having a plurality of emitter means spaced along its length, each emitter means configured and arranged to, in use, emit a signal substantially perpendicularly to the said arm to be received by a corresponding receiver means on the second arm, and a control console, the control console containing: control means to, in use, control the rate, strength and regularity of the signal emitted by each of the said emitter means, collection means to, in use, collect data from each receiver means as to the existence or absence of receipt of a signal, processor means to process data received from the control means and the collection means and determine the height of any plant matter traversed by the plant matter sensor at predetermined intervals, and storage means to store the plant matter height data generated by the processor means for subsequent download or analysis: and wherein the support frame includes: attachment means for attaching the support frame to a towing vehicle, sensor mounting means for mounting the sensor and maintaining it in a substantially vertical orientation, and ground engagement means to maintain the sensor at a predetermined level above the ground during operation; the support frame further including a pair of spaced apart deflector guards configured and arranged to, in use, present a defined bandwidth of pasture passing between the sensor arms; the defined bandwidth of pasture being narrower than space between sensor arms. a pair of parallel spaced apart substantially vertically disposed sensor arms, a first of said arms having a plurality of emitter means spaced along its length, each emitter means configured and arranged to, in use, emit a signal substantially perpendicularly to the said arm to be received by a corresponding receiver means on the second arm, and a control console, the control console containing: control means to, in use, control the rate, strength and regularity of the signal emitted by each of the said emitter means, collection means to, in use, collect data from each receiver means as to the existence or absence of receipt of a signal, processor means to process data received from the control means and the collection means and determine the height of any plant matter traversed by the plant matter sensor at predetermined intervals, and storage means to store the plant matter height data generated by the processor means for subsequent download or analysis: and control means to, in use, control the rate, strength and regularity of the signal emitted by each of the said emitter means, collection means to, in use, collect data from each receiver means as to the existence or absence of receipt of a signal, processor means to process data received from the control means and the collection means and determine the height of any plant matter traversed by the plant matter sensor at predetermined intervals, and storage means to store the plant matter height data generated by the processor means for subsequent download or analysis: and wherein the support frame includes: attachment means for attaching the support frame to a towing vehicle, sensor mounting means for mounting the sensor and maintaining it in a substantially vertical orientation, and ground engagement means to maintain the sensor at a predetermined level above the ground during operation; attachment means for attaching the support frame to a towing vehicle, sensor mounting means for mounting the sensor and maintaining it in a substantially vertical orientation, and ground engagement means to maintain the sensor at a predetermined level above the ground during operation; the support frame further including a pair of spaced apart deflector guards configured and arranged to, in use, present a defined bandwidth of pasture passing between the sensor arms; the defined bandwidth of pasture being narrower than space between sensor arms.