Onion Crop Variability

April 2018

Identifying the causes of variability in onion crops at the Centre for Land and Water's MicroFarm in Hastings

A three-year study in its final season is looking at growth variability within onion crops and practical ways for growers to locate and monitor where variations are occurring, and identifying possibilities for management changes to make the best of the situation in any given season. The study is sited on the MicroFarm at the Centre for Land & Water, near Hastings, and is funded by Onions NZ and the Sustainable Farming Fund.

The LandWISE MicroFarm is 4ha of arable land in the middle of the Heretaunga Plains. Its paddocks are small but allow conventional tractors and equipment to operate with best practice. This scaled-down version of a cropping farm is being used to develop and demonstrate new sustainable techniques that maximise food production, soil care and input efficiency. A key aspect is that it brings together farmers, technologists, service providers and researchers who oversee the research programme and determine strategic management.

In recent years research projects have largely revolved around process crops such as sweetcorn, beans and peas. Dan Bloomer, part-time manager of LandWISE Inc., says that some years ago they had a small patch of onions and to get a good look at the crop he climbed up the irrigator.

“When you get 7m up in the air and look down is very different to what you see walking around, and I saw massive variability throughout the crop,” he says.

“I could understand some of it but a lot more I couldn't explain. A colleague was playing around with imaging and smart phones and he made an application where we could drive up and down the rows and map the percentage canopy cover. I talked about it with the Onions NZ research manager who thought it would be quite useful and could lead to greater understanding of variability in onion crops.”

“We then partnered with Plant and Food Research with support from industry peak body Onions NZ Inc and the Sustainable Farming Fund to do a project aimed at studying that variability.”

Dr Bruce Searle, a crop scientist with Plant & Food Research, designed a research approach to get the data they needed to make some practical applications.

“We wanted to figure out where the variability comes from and how much of it is something that a grower can control. So we looked at the different factors that might influence variability and worked through the contribution of each to the overall variability,” he says.

“A lot of it comes down to individual plants growing at different rates – something that the grower can't do a lot about. However, factors that influence getting good crop establishment are critical to reducing variability, and once the crop is up you can look at poor performing areas within in the crop. The tool that Dan has been working on captures that information so that you can map the field and make some decisions.”

The system the team has developed takes smartphone images of the crop and analyses them to help determine whether the cause of the variability is poor establishment, and therefore a poor population of plants, or whether establishment is ok but growth is being limited in that area by some other factor. Along with image analysis, modelling takes into account the climate, rainfall and the soil nutrient status to determine whether the growth is less than the expected potential.

“You could have good growth and the plants are quite large but there are fewer than there should be in that area so that your overall the yield is going to be low and the variability is going to be high because you are going to have a whole bunch of really big plants. Reducing fertiliser application on that area could keep plant size more in line with the rest of the crop,” says Bruce.

“On the other hand the plant population might be as expected but growth is below the potential and so you need to figure out why. We know that soil compaction and drainage are key issues to start looking at first.”

The system of image analysis makes it easier for growers to assess their crop and gives the possibility of taking remedial action in the current season rather than waiting until the next one. Dan says that after nearly three years their techniques allow them to do a good job of mapping paddocks and identifying different zones.

“We establish plots in the zones and use smartphone imaging to determine the percentage in the plots. We also count the population in the plots, and put the data into a website. And it might tell us that in a particular zone the population is lower than we are aiming for and it will restrict yield, or the population is okay but growth isn't any good, or they are both fine and you don't need to do anything, or they both suck so you got a lot of work to do,” he says.

“We are now reasonably comfortable that imaging lets us create accurate yield maps at the end of the season, and we are working on using it to make yield predictions on the way through the season.”

“The system is a good means of identifying problem areas. What it can’t tell us is why plants aren't establishing or growing well but we know that a big chunk of it is genetic and we know that another critical aspect is the speed of emergence of the new seed. We also know that plant number per hectare is important but plant spacing is not.”

The question of how much fertiliser to put on each zone is also being examined. Dan explains that where fewer plants have emerged and the canopy area is reduced they have a trial of half-rate versus full rate application, the reasoning being that fewer plants will need less fertiliser.

“Another question we are looking at relates to sustainability. Some of the growers put all their fertiliser on very early when the onions have not long emerged. Plant & Food have done some research indicating that plants use only a third of their total nitrogen requirements up to the point at which they start to bulb and the rest gets taken up after that,” he says.

“The soil generally has enough nitrate for initial growth so we have another trial where we are not putting fertiliser on until bulbing starts and comparing that with conventional early application. My argument is that if we put fertiliser on early and then have heavy rain it is going to take the nitrate down below the plant roots, which don’t go very deep.”

“The other aspect is that if you give onions too much nitrogen they don't store well, so if you have an area of low canopy maybe you need to put much less nitrogen on to avoid storage problems.”

A quick and cheap test for soil nitrogen is available and is similar to a swimming pool test strip. Bruce recommends that growers do this test before they apply nitrogen to the zone, and in many cases N application could be delayed until further measurements indicate soil N is reducing, or until bulbing. The test involves dipping the end of a test strip in soil water and matching the resulting colour to an indicator chart.

The 2018 season is the final one for the project. It has included mapping paddocks in commercial crops in Pukekohe and Canterbury as well as in Hawkes Bay. This helps to involve a greater number of growers and so spread the information, a factor that Dan says is vital to the project’s success.

“We want to engage the whole industry all the way through otherwise it’s possible that we might end up with a ‘tremendous achievement of the wrong thing’. I think it is quite possible to wander off on a track that is not meaningful to growers, whereas if growers are engaged from day zero and stay engaged I find that the conversations are very rich and they will point out problems help frame that project so that it will be useful to them,” he says.

“The other thing that we have learnt from this project is that being deeply engaged with scientists is very valuable because they think differently and have valuable insights. This inclusive approach helps ensure that we end up doing work that really bridges the gap between science and farm.”

www.landwise.org.nz and http://www.landwise.org.nz/microfarm/