This project addresses the environmental and economic pillars of sustainability through simulated modeling of varying beef cattle diets, which allows insight into understanding the relationship between resource availability and how to best match with local environments. It also determines the economic impact through understanding genetic potential of cattle to different feedstuffs and what results in highest economic output. The simulations were parameterized to replicate different land regions in the Great Plains and varying cattle genetic potentials within those regions. Further, the resource inputs of diets including corn products were compared to diets including grain sorghum products in regions where grains are often fed to cow-calf herds. The objective of this project was to use computer generated simulations of a 100 head cow-calf operation to determine land, water, and fertilizer requirements as well as methane emissions for various regional beef production scenarios.
A stochastic simulation model was utilized to simulate a 100-head cow-calf operation over a 25-year period. The program simulated herds in 74 land use regions in the Great Plains, which varied in genetic potential for mature weight and milk production as well as primary grain source (sorghum vs. corn), resulting in 444 unique scenarios modeled. Body size (large, medium, small) and lactation potential (high and low) was parameterized using data from regional surveys of cattle producers. The cattle were assumed to be grazing from May 1 to October 31 with supplemental feed as needed. The stocking rate for each scenario was based on observed stocking rates and scaled to match mature body weight. From November 1 to April 30, the cattle were assumed to be delivered enough of a ration of hay and grain to maintain a body condition score of 5. The supplemental and delivered rations were formulated to be representative of common feedstuffs in each region. The amount of each feedstuff required to maintain each animal at optimal body condition was calculated. For sorghum-based rations, the amount of corn products was substituted by a net energy equivalent amount of grain sorghum products (whole grain or dried distillers grains).
Average yield for each feedstuff was found for a representative county located in each land region. The as-fed weight of the feedstuffs was divided by the yield of the corresponding feedstuff in that region to calculate the hectares of land required for each scenario. For by-product feedstuffs, land used to grow the original crop was scaled by the percent mass of the byproduct compared to the mass of the original crop. Grazing land was determined by multiplying the number of animals of each class (replacement heifers, bred heifers, and mature cows) by the stocking rate of each class. Total land was the sum of land required to grow feedstuffs and grazing land. Irrigation water needs were found by subtracting rainfall measurements from average crop water needs as determined by the Blaney-Criddle method. Total irrigation water was found by multiplying irrigation needs by the amount of cropland allocated to growing feedstuffs. Drinking water was estimated for the herd each month by adjusting a baseline water intake by body weight, peak lactation, and monthly temperature in each scenario and multiplying by the number of cattle in the herd. Total water use was calculated as the sum of irrigation water and drinking water. Fertilizer use was calculated by inserting yield estimates of each feedstuff, including forages, for each scenario into soil nutritional requirement equations. It was assumed that no nutrients were in the soil and all nutrients had to be applied, so estimates reflected differences in crops but overestimated the total amount of fertilizer that would need to be applied in a specific region. The fertilizer estimates were then multiplied by the land allocated to each feedstuff to find the total amount of nitrogen, phosphorus, and potassium required. The gross energy of each diet was multiplied by dry matter intake of each diet. The gross energy intake was inserted into the Intergovernmental Panel on Climate Change Tier 2 methane estimation model to find the kilograms of methane produced in each scenario in the average year. The land, water, fertilizer, and methane estimates were generated twice for each scenario: once for diets using corn products and once for diets using grain sorghum products.
When comparing genetic potentials, large, high milking (LH) cattle generally exhibited the largest environmental footprint because they often required the most delivered ration, followed either by medium, high milking (MH) cows or large, low milking cows (LL). Small cows with low milking potential (SL) required the least amount of delivered ration. The small, high milking (SH) cows required the most supplement while grazing, likely because they were incapable of consuming enough forage to maintain an optimal body condition score. High milking cattle required more land for growing feedstuffs than low milking cattle. However, when grazing land was included, larger cattle required more total land than smaller cattle, even if milk production was lower. Total water use followed the same trend as total land use. LH animals needed the most water, followed by LL, MH, medium, low milking cattle (ML), SL, then SH cattle. However, most of this trend was driven by irrigation water demand, rather than the increase in drinking water required for cows that drank more or had higher milking potential. In addition, for land regions where cow-calf operations were fed grain, methane production followed similar trends as land and water use. However, where no grain was fed, the LH animals still produced the most methane but were followed by MH cows, then either SH cows or LL cows, depending on the land region. When comparing corn-based diets to grain sorghum-based diets, corn diets required less land and irrigation. Though sorghum requires less water than corn, it is less energy-dense and yields are lower, so this reduction offsets the gains seen in water efficiency of the crop. Changes in genetic potential for yield of sorghum through investments in research and changes in management practice may alter this result in the future. While corn-based diets have advantages in land and water use due to increased yield, sorghum-based diets required less potassium in all land regions and required less nitrogen and phosphorus in most land regions.
Consideration should be given to matching both cattle’s genetic potential and the nature of feedstuffs to the regional environment to reduce environmental impact while maintaining profitability. While practices differed between land regions, some overarching trends can be identified from this study. First, the environmental impacts of any cow-calf operation were heavily dependent on location. Next, LH cattle had the largest gross environmental footprint because they required the most inputs to maintain. Larger animals produced more methane than smaller animals by virtue of consuming more total feed, while higher milking animals had a greater need for crop land because they require more supplement. The water footprint for beef was mainly driven by the irrigation required for feedstuffs. Furthermore, feedstuffs, like cattle, should be matched to the environment. If the beef industry were to make the shift to using solely grain sorghum products, it is possible crop production practices would change and sorghum yield would increase. If that occurred, and provided additional water would not be required to match the new sorghum yield, the overall water footprint of beef could decrease.