Crop selection and data sources
Selection of frost resistant crops for temperate countries was based on the grading in Table 129. For these crops we then used food composition data (dietary energy and protein) and crop yield data to complete the table of data inputs (Table 2). In most cases New Zealand specific data were available, but where not we prioritized the use of relevant Australian data, then North American data, and then European data.
Table 1
Relative frost resistance of temperate crop species
(source:29, with minor adaptations)
Damaging temperature range | General hardiness rating | Food crops and grasses for livestock |
+ 5 to 0°C | Chill and frost sensitive | Tomato, cucumber |
0 to -2°C | Very frost sensitive | Potato foliage, French beans, maize, melon |
-2 to -4°C | Frost sensitive | Fruit blossom, grapevine in bud burst, cereals in ear, cauliflower curds, broccoli spears, asparagus spears, spring peas, spring lettuce |
Crops which are included in the analysis of this current study – data permitting |
-4 to -7°C | Moderate frost hardiness | Winter oats*, spring cereals, cauliflower and broccoli leaves, kale*, white and spring cabbage, sugar* and fodder beet*, onions, swede*, spring canola*, winter lupins*, carrots, winter lettuce, parsnips Grasses: Italian ryegrass, red clover, alfalfa (lucerne) |
-7 to -10°C | Reasonable frost hardiness | Winter barley*, winter canola*, winter field beans*, winter linseed*, savoy cabbage, spinach, rhubarb crowns Grasses: white clover |
-10 to -15°C | Good frost hardiness | Winter wheat* # Grasses: perennial ryegrass |
Colder than − 15°C | Very frost hardy | Winter rye* #, dormant deciduous plants including dormant grapevine Grasses: timothy and fescue grasses |
*These crops can be used as livestock forage crops, but they can also be used to feed people.
# There are some cultivars that are even more cold tolerant e.g., down to − 24°C for the winter wheat cultivar Norstar and − 33°C for the winter rye cultivar Puma27.
Table 2
Nutritional data and crop yield data used in the analysis
Food crop (from Table 1) | Dietary energy (kJ per 100g) | Dietary protein (g per 100g) | Crop yield in tonnes per hectare (range)* | Data sources, comments [database code for New Zealand food composition data30 (all data are specifically for New Zealand unless otherwise indicated) |
Vegetables | | | | |
Broccoli | 140 | 3.8 | 16 | Nutrients: [X1020], fresh, raw Yield: Based on winter planted |
Cabbage (white) | 108 | 1.2 | 68 | Nutrients: [X1102], green drumhead, fresh, raw Yield: Based on winter planted |
Cabbage (savoy) | 130 | 1.7 | 44 (29 to 59) | Nutrients: [X1260], fresh, raw Yield: Based on the range from a European study indicating 29–31 tonne per hectare (t/ha) without fertilizer and 56–59 t/ha with highest fertilization (fresh matter yields)31 |
Carrots | 156 | 0.6 | 120 (70 to 170) | Nutrients: [X1114], fresh, raw Yield: Based on “process carrots” (not “table carrots”) |
Cauliflower | 79 | 0.8 | 33 | Nutrients: [X1128], fresh, raw Yield: Based on winter planted, New Zealand data32 |
Field beans | 143 | 2.1 | 25 (20 to 30) | Nutrients: [X1108], green runner or dwarf, seeds with pod, fresh, raw Yield: Based on weight in the pod with the assumption that “90% of the pods are removed from the field”, New Zealand data32 |
Kale | 168 | 4.6 | 15 (12 to 17) | Nutrients: [X1163], fresh, raw Yield: Based on New Zealand forage crop data and covers “short”, “intermediate” and “giant” cultivars.33 This yield range is for dry weight and so there is likely that the nutrient energy and protein (as presented in this table) are slightly under-estimated |
Lettuce | 57 | 1.5 | 24 (18 to 30) | Nutrients: [X1265], green lettuce, assorted varieties, raw, fresh Yield: Based on the range covering winter (18 t/ha) and summer (30 t/ha) lettuce, New Zealand data32 |
Onions | 130 | 1.4 | 70 (60 to 80) | Nutrients: [X1130], brown, fresh, raw Yield: Based on “fresh marketable yields” of the Pukekohe long-keeper onion (which are 10–13% bulb dry matter), New Zealand data32 |
Parsnips | 235 | 1.0 | 25 (20 to 30) | Nutrients: [X1097], raw Yield: Based on the range from Canadian data34 |
Rhubarb crowns | NA | NA | NA | |
Spinach | 75 | 2.5 | 20 (15 to 25) | Nutrients: [X1045], English, raw Yield: New Zealand data32 |
Sugar beet | 180 | 1.6 | 67 | Nutrients: US data,35 [FDC ID: 169145], beets, raw Yield: Based on the European Union (27 country) average for sugar beet in 2020.36 Yield does not include potentially edible leaves. This European estimate is compatible with an estimate from a 1980s study for the Waikato in New Zealand which estimated a commercial 60–70 t/ha sugar beet yield, with slightly higher yield than fodder beet37. |
Swede | 125 | 0.8 | 33 (25 to 40) | Nutrients: [X1167], peeled, fresh, raw Yield: Based on Australian data for crops for human consumption38. Of note is that this yield does not include the green tops which can also be eaten by people. New Zealand forage crop data indicate a yield of 12 to 16 t/ha but this is for dry weight39. |
Grains | | | | |
Barley | 1300 | 9.2 | 7.0 | Nutrients: [E1], wholegrain, raw Yield: Based on a 5-year average for New Zealand reported in 202040. Barley is mainly planted in mid-spring in New Zealand41. |
Canola | 2686 | 18.6 | 3.1 | Nutrients: Based on animal feed data (wet weight) for Europe42. We note some compounds in canola meal can impede uptake of some micronutrients (e.g., glucosinolates and phytates) and so additional processing may be desirable for optimal human nutrition43. Yield: Based on the European Union (27 country) average for 202036. A New Zealand farm has reported an “unofficial new world record” yield of 6.3 t/ha.44 |
Linseed | 1890 | 18.4 | 2.8 (2 to 3.5) | Nutrients: [Q1027], linseed or flaxseed, brown or golden, whole, dried, raw Yield: Based on the range from good dryland cropping soils in New Zealand (2-2.5 t/ha) to irrigated soils (3-3.5 t/ha)45 |
Lupins | 1550 | 36.2 | 2.2 | Nutrients: US data,35 [FDC ID: 172423], mature seeds, raw Yield: Based on Western Australia data46 |
Oats | 1586 | 13.2 | 7.9 (5.7 to 10) | Nutrients: Based on US data35 [FDC ID: 2343973], raw oats Yield: Based on Southland, New Zealand, yields47. These yields are much higher than the European Union (27 country) average of 3.3 in 202036. |
Rye | 1230 | 11.0 | 4.3 | Nutrients: [E29], wholegrain flakes, raw Yield: Based on the European Union (27 country) average in 202036 |
Wheat | 1400 | 13.4 | 9.9 | Nutrients: [E35], wholegrain, raw, North Island, New Zealand Yield: Based on New Zealand data for 2020 (and very similar to a 5-year average).40 However, a New Zealand farm has reported a world record of 17 t/ha48. An estimated 75% of wheat in New Zealand is planted as “winter wheat” (i.e., autumn planting)41. |
Grass-fed livestock products** | |
Cow’s milk | 278 | 3.5 | 5.4 (milk) | Nutrients: [F1086], milk, cow, whole 4% fat, fluid, non-homogenized Yield: Using a New Zealand average of 650 kg milk solids per ha (using the latest year for data in Fig. 15 in:49). Converted back into whole milk in the adjacent column by an adjustment of 8.3 (100/12) given that ~ 88% of milk is water50. |
Lamb | 581 | 20.6 | 0.104 (meat) | Nutrients: [M1150], lamb, forequarter & hindquarter assorted cuts, separable lean, raw Yield: Using a New Zealand average of 104 kg meat per ha (using the latest year for data in Fig. 17 in:49).*** |
Beef | 746 | 20.0 | 0.310 (meat) | Nutrients: [M1220], Beef, forequarter & hindquarter assorted cuts, separable lean & fat, raw Yield: Using a New Zealand average of 310 kg meat per ha (using the latest year for data in Fig. 16 in:49). |
* Yields are typically for the “marketable yield” (not fresh biomass in the field), for one crop planting and for New Zealand32, unless details for another country are stated. Where a range is reported, the analyses used the mid-point of the range.
** Yields for these livestock products could be slightly higher if the analysis was expanded to estimate the meat from dairy cows (at the end of their milking lives), and if all edible parts of the animal carcasses were included (e.g., organ meats, the marrow inside of bones, and bone meal).
*** In disaster circumstances production of lamb (largely for export markets) could shift to the more efficient production of mutton from mature sheep carcasses. Mature sheep also have the other benefit of providing wool.
Business-as-usual dietary intakes of the New Zealand population
The estimated dietary energy intake of the entire New Zealand population has previously been estimated at 44.4 billion kJ per day, equivalent to 8686 kJ per person per day20. The same approach was also used to calculate protein intake, as per Table 3.
Table 3
Estimated daily protein energy intake of the total New Zealand population
Population group | Average estimated daily protein intake in grams (nutrition survey data51,52) | Population size (Q4 2021 estimates53) | Total per day (tonnes) |
Adult men (15 + years) | 102 | 2,041,970 | 208 |
Adult women (15 + years) | 71 | 2,105,180 | 149 |
Children* (< 15 years) – males | 62 | 496,930 | 31 |
Children* (< 15 years) – females | 52 | 470,720 | 24 |
Total | – | 5,114,800 | 413** (equivalent to 81g/person/day) |
* We used the protein intakes for the 5-6-year-old-age-group, which is likely mid-range for the 0–14 year group. That is, the intakes for the < 5 year age-group were not collected in the survey data and for the 11–14 year age-group the intakes were boys: 88g/d; and girls: 66g/d52.
** This total might be slightly below the ideal for planning purposes since some of the adult survey respondents reported food insecurity, some people may have been dieting to control their weight at the time of the survey, and because of under-estimation of food intakes associated with the use of food diaries54. Nevertheless, the intakes for adults in New Zealand are relatively high compared to the dietary recommendations of 64 g/d for men (63% of current intakes) and 46 g/d for women (65% of current intakes) (with recommendations being for the 19-70-year-age-group)55.
Optimization
We aimed to identify the minimal amount of cropping land to provide sufficient frost resistant crops to feed the entire New Zealand population. For the mathematical optimization we used linear programming conducted with Excel Solver (using the “Solver LP” method). The objective function was the minimization of the total cropping land required (ha) and the constraints in the base case were to achieve ≥ 8686 kJ/day of dietary energy per person and ≥ 81 g/day of dietary protein per person. These constraints were modified in various scenarios (see below) and comparisons were made with the total area of crop land used in 2019 in New Zealand (132,717 ha in horticulture and 487,763 ha in grain)56. More specifically, the level of current frost resistant crop production was assessed in terms of its capacity to feed the population under the different scenarios.
Scenarios for nutrition and crop selection
Reduced dietary energy (10% less) and protein (35%) requirements were considered in Scenario A. This was on the basis that many people could probably tolerate slightly lower energy intakes in a disaster situation, and the current protein intakes are relatively high compared to recommended levels (see Table 3 footnotes).
Scenario B assumed 50% of both energy and protein intakes from frost resistant foods and the rest from other food sources. The latter could include:
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Production of some frost sensitive crops (e.g., potatoes) that could potentially be grown if there were no out-of-season frosts in warmer parts of the country and/or in greenhouses.
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Production of grass-fed livestock products (e.g., dairy and meat from grass-fed livestock near towns/cities or railway networks). Persisting grass-fed livestock production is highly feasible during a nuclear winter given that all the major pasture grasses grown in New Zealand are frost resistant (e.g., the ryegrass and clover57 included in Table 1).
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Production of frost resistant vegetables and poultry from home gardens, urban community gardens, and Māori community gardens58.
Nuclear winter scenarios
For each of the above scenarios we considered a range of nuclear winter scenarios (Table 4). The wide range of these reflects that there is still much uncertainty concerning the global impacts of nuclear war and if substantial “nuclear winter” type impacts would even occur59. Nevertheless, these scenarios generally assume soot injection into the stratosphere following nuclear weapon explosions on targets in the Northern Hemisphere16. This soot is assumed to then block sunlight which then results in lower surface air temperatures as well as reduced precipitation levels, both of which impede food crop production in both hemispheres16. In all these scenarios we assumed a worst-case situation of a complete end to New Zealand’s trade with other countries (including Australia) for both exports and imports. This was also the approach taken in previous New Zealand research20 60. We also ignored stockpiled food awaiting export that could be diverted to domestic use as this would only be temporarily and is predominantly dairy products (e.g., milk powder) and frozen meat.
Table 4
Four nuclear war and nuclear winter scenarios considered in this study
Nuclear war scenario | Scale of impact of a nuclear winter | Estimated impact on agricultural production in New Zealand |
A nuclear war in the Northern Hemisphere, not otherwise specified (NW0) | The war in this scenario is assumed to result in no nuclear winter impacts | Zero from no significant changes to sunlight, temperature, precipitation, and ozone levels |
A war between India and Pakistan (NW1) | We used the lower end impact (5 Tg) of the estimated 5 to 47 Tg range of stratospheric loading of soot from a nuclear war. This is from a major modelling study published in Nature Food in 2022 by Xia et al16. | An 8% reduction in major food crops and marine fish* 16 |
A war between NATO and the then Warsaw Pact (NW2) | We used the impacts from a New Zealand Planning Council study of a 5000 to 6000 megatonne war in July (Northern Hemisphere summer)60. This was assumed to result in a spring temperature reduction in New Zealand of 3° C, a 2° C reduction in summer and a 1° C reduction for another 18 months. | A 28% reduction (mid-point in the estimated 19–36% reduction in pasture growth in year one)** 61 |
A war between Russia and the US and its allies (NW3) | We used the highest value (150 Tg) of stratospheric loading of soot used in the work by Xia et al16. | A 61% reduction in major food crops and marine fish* 16 |
* This study by Xia et al estimated food energy production for New Zealand as part of a global analysis using data for major food crops (maize, rice, soybean and spring wheat) and marine fish averaged in year two. For modelling parsimony, we used these specific reductions for across-the-board food production, even though grass growth for livestock production may be less impacted than crop production. This − 8% value from Xia et al 5 Tg scenario compares with − 12% for maize and wheat in year 4 (and a -5% average for years 1–5) in an earlier study (i.e., Jägermeyr et al: Table S3)15.
** This New Zealand work61 estimated pasture dry matter production impacts from nuclear war for Waikato, Canterbury and Southland regions with reductions in year one ranging from 19–36%. For year two the range was reductions from 11–17%. It has also been estimated for New Zealand that a 3° C decline in temperature would delay the maturity of wheat crops in Canterbury by about 40 days62. This would probably not be a problem but a 20% decline in solar radiation would result in a probable decline in yield of 15%. Also noted was that in Southland a 3° C decline in temperature might actually prevent maturation of grain crops62.