Estimating the yield of a brassica crop is critical for allocating the correct break size and animal allowance.

 

Fresh weight

Joining both ends of a 3.54 m length of alkathene will give a circle with an area of 1 m2. This circle should then be placed over representative areas of the crop. Everything within the circle should be cut, placed in a bag and weighed to give the fresh weight (remember to subtract the bag weight).
For bulb crops sown in rows it is more appropriate to harvest part of a row. Row width will determine length sampled, e.g. for 60 cm wide rows, a 1.67 m row sample length will give an area of 1 m2.

 

In all situations

Sampling should be repeated at least 5 times across the paddock to gain an average fresh weight. The more variable a crop the more samples need to be taken.
The fresh weight is then multiplied by 10,000 to convert from kg/m2 to kg/ha (10,000m2 = 1 ha).

 

 

Dry matter %

Three samples per crop should be taken to determine the DM %. For each sample take 2 or more whole plants (representative of the crop) and chop into small segments. Weigh the sample to determine fresh weight and then dry at 60-90ºC for 48 hrs, until the weight stops falling. Then weigh the dried sample.
The dry matter % can be calculated by dividing the dry weight by the fresh weight.
E.g. Fresh weight of sample = 112g
Dry weight of sample = 16.8g
Dry matter % = 16.8g ÷ 112g
= 15%
DM testing is offered as a service by some laboratories.

 

Crop yield
(kg DM / ha)

Once the fresh weight yield (91,000 kg/ha) and the dry matter % (15%) are known the crop yield in kg dry matter per hectare can be calculated.
Crop yield = Fresh weight x Dry matter %
(kg DM/ha) (kg/ha) (DM%)
= 91,000 kg/ha x 0.15 (15%)
=
13,650 kg DM/ha