As I shifted the sheep from one grazed section of the field to the next luscious cover crop patch, they moved in threes, filling the width of the corridor.  They became more frantic to get to wherever the sheep in front was going, and the electric netting that formed the corridor leaned precariously on its side. What had seemed like a structurally sound enclosure was now looking very… open. I held my breath; all I could do was wait. The continuous and fluid-like nature of their motion convinced me there was no way a single, discrete sheep would deviate from the flock’s path. It was only when there was a slowdown in the flock, because the sheep in the new section started grazing, that I saw one single aberrant ewe turn her head towards the pushed down fence. I could almost hear her thought process “Huh, what’s over here? …looks pretty open… “. I turned to my lab mate, Alex, who was helping me, and our eyes widened in unison, but we both couldn’t utter a word. At risk of adding more commotion to the situation, I felt the best approach was to stay still and quiet. Luckily, the aberrant ewe noticed that the flock was in motion again, straightened her head and continued with the rest of the flock. Thank goodness for herd behavior! Alex and I both let out a collective sigh and could speak again. When all the sheep had moved over to the next section, the chaos subdued and we paused in a moment of stillness, appreciation and reflection.

I am a Soils and Biogeochemistry graduate student in Dr. Amélie Gaudin’s Agroecology lab. In general, agroecology is the science of viewing agricultural systems through an ecological lens, though it can also be seen as a movement and a set of practices [1]. Agroecological practices include those that minimize soil disturbance and increase biodiversity in a system.  In Dr. Gaudin’s lab, we study how different management practices in agricultural systems affect the provisioning of ecosystem services and productivity. We look at interactions between plants, animals, and soil and try to understand the underlying mechanisms behind management practices that drive change in a system. Specifically, I am researching integrated crop-livestock systems, or systems in which crops and livestock are produced on the same piece of land [2].  My field work over the 2020 spring season included two grazing events at Russell Ranch Sustainable Agricultural Facility to investigate how sheep (Ovis aries) grazing of a winter cover crop impacts soil health, subsequent tomato yield, and potential food safety issues. Cover crops are plants that are grown for their benefits to the soil, but are not harvested for any production purposes. In my experiment, annual ryegrass, field pea and vetch were planted as winter cover crops that grow during the rainy winter. These plants are incorporated into the soil before processing tomatoes (tomatoes grown for making canned tomato products) are planted in spring. As of 2019, there were 235,000 acres of processing tomatoes in California, equal to about 3% of all cropland in the state [3]. These tomato fields are typically left fallow (bare, without plants growing) throughout the winter months. However, planting a cover crop and grazing it can transform a barren landscape into a more biodiverse, multipurpose agroecosystem. In addition to greater biodiversity, grazing on land that would otherwise be unproductive increases the land use efficiency and productivity of a piece of land by producing a meat product as well as a plant product. With increasing urban development trends that are leading to encroachment onto farmland and more extreme weather events due to climate change, Californian farmers, ranchers, and land managers need creativity in managing their agroecosystems; integrated crop-livestock systems could be a good tool for navigating such complex times. However, before recommending this practice, first we need data to understand how grazing impacts soil health, crop yield and food safety.

For now though, those big questions could wait. Being out in the field with the sheep was invaluable and my mind was always ruminating (pun intended) with thoughts on sheep behavior and how their behavior would impact the plants and subsequently the soil. In my experiment, I have three main winter soil management treatments: fallow, ungrazed cover crop, and grazed cover crop. While the grazed and the ungrazed cover crop treatments began with the same seed mix in November 2019, by spring, after grazing, the differences were stark (see Figure 2). This was due to the differences in how the cover crop species respond to grazing. The ryegrass responds well to grazing, while the field pea and vetch (both legumes that can fix atmospheric nitrogen and add it to the soil), typically don’t regrow after being grazed. The first time the cover crop was grazed, there was a balance of ryegrass (the sheep equivalent to kale- good for you, but not too tasty) to legumes (the sheep equivalent to candy- a yummy treat), that was essentially all consumed. By the time the sheep were moved back into my experimental plots again, the ryegrass had regrown easily after the graze, but the legumes were very sparse. When the sheep were turned out into the field for the second time with ample grass forage this time, they first selectively ate the few legumes that managed to regrow. The sheep’s own selective pressure had created the human equivalent to a scavenger hunt for treats! Meanwhile, in the ungrazed cover crop plots, there was about 50% ryegrass to 50% legumes throughout the duration of the winter and spring. With no selective pressure from the grazing, the cover crop species maintained the ratio at which they were seeded. 

Although managing an integrated crop-livestock trial can be quite hectic (keeping the sheep grazing in the correct plots, moving fence to the next section etc.), there are moments of calm that bring great joy. My little plot at Russell Ranch seemed to be teeming with all sorts of life. First of all, the sheep were absolutely jovial to be frolicking in the cover crop! One sheep, dubbed “Venti” by the undergraduates who worked with her, was so calm and curious. She often walked right up to us with interest, sniffing the ruler I was using to measure cover crop height (see Figures 3 & 4). Red-winged blackbirds (Agelaius phoeniceus) perched on the few tall, wild mustards that managed to take hold in the cover crop, a tall vantage point overlooking their domain. I spotted an owl soaring over the ungrazed cover crop during my “sheepover” one night. A pipevine swallowtail butterfly (Battus philenor) fluttered near the flowers of the ungrazed cover crop. Finally, while enjoying the sun on the last afternoon of the second graze, Dr. Alda Pires, a Cooperative Extension Assistant Specialist with a focus on Urban Agriculture and Food Safety and the Co-PI on this project with Dr. Gaudin, pointed out a dog-like animal on the levee behind the field. It was only about 20 pounds and had a grey-reddish hue. Seeing this coyote (Canis latrans) was a grand culmination of my field season, and a great reminder about the many trophic levels that exist within an ecosystem.

Figures 3 & 4 – Curious Venti sniffs my ruler as I am measuring cover crop height and poses for the camera

On the last day of the second graze, while walking back from observing the sheep on top of the neighboring levee, I took inventory of all the sheep. Suddenly, I noticed one sheep was rolled over on her back, her legs splayed towards the sky. Other sheep approached, giving her a sniff, as if to say, “What are you doing on your back?!” I looked at her ear tag: #7008. My mind started racing. I wondered how she could be bloating on this fresh ryegrass when during the first graze, the forage had higher nitrogen content (due to the greater proportion of legumes) and the sheep were fine. When ruminants (such as sheep) eat forage, it is broken down by microbes in their rumen, or gut. Normally, the carbon dioxide that the microbes produce in the rumen can be expelled by the sheep through burping. However, if the forage is too high in nitrogen, a layer of froth is formed in the rumen that locks the gas in. This can lead to a dangerous condition known as “frothy bloat” when the stomach of the ruminant is inflated and can lead to death if it is too severe. 

I pulled out my phone and called Alda, who was further down the field and just out of yelling distance, “What do you think Alda? Is it bloat?”.

“Absolutely, but she is only bloating because she fell over into the furrow and can’t get up. That is gassy bloating, simply because she can’t burp.” Alda explained. “It’s easier to fix than frothy bloat, the type of bloat sheep get from eating forage that is too high in nitrogen.”

We hoisted #7008 up so she could produce some burps and then she joined the rest of the flock, back to normal in a matter of minutes. The mortality of the sheep suddenly stuck me as I realized that these sheep were not only dependent on us for food, water, shelter, but also for the occasional flip upright. Grazing on cropping land means dealing with uneven terrain that the sheep may not be used to. Cropping systems in California use beds, flat raised sections of soil where the plants are grown, and furrows, indentations in the soil for the tractor tires and where no crops are grown. The furrows are a tripping hazard for the sheep, and they may not be able to get up on their own accord if they fall and land with their back exactly in the furrow, like #7708 had done. Recognizing the risks of grazing sheep on beds with perilous furrows is important when land managers are considering implementing grazing and their frequency of checking their flock.  

After the sheep had grazed the appropriate plots, their work was done, but the field work challenges were far from over; it came time to get the flock back to the UC Davis sheep barn. We had to break the flock up into multiple groups, because they wouldn’t all fit into the trailer at once. The fluid-like nature of the sheep I had seen in the fields was no longer reassuring. How could we break the flow of the motion in order to separate the sheep flock into groups? Mark Rubio, Senior Superintendent of Agriculture of the UC Davis Department of Animal Science, had a few tips for us. We had to very calmly walk alongside the sheep until we had about 1/3 of the flock on one side of us and 2/3 on the other side. Then it was a matter of pushing the smaller group further down the alley so we could pick up the fences and use them to close the two groups into separate enclosures. The main struggle was keeping the two groups apart. I had to engage in an intense staring contest with the sheep at the front of the flock while standing with my arms extended up and out and with my legs in a wide stance, like a human starfish, while someone else moved the fence to create the groups. The sheep with tag #4237 was especially spirited. She liked to be at the front of the flock, squaring off with me. Any accidental falter, or a glance over my shoulder to check out the other group and #4237 would bolt around me starting a domino effect of all the others to follow. Suddenly our 2 groups merged together and it was back to square one. After finally separating the first group and loading them up into the trailer, Alda, her interns, and I were left separating the remaining sheep into two groups. We managed to get the group split into two groups, one of 35 and one of 20. To attempt evening out the two groups, Alda stood holding the smaller group and I allowed a few sheep to think they were getting away with mischief by slipping over into her group. Success! We had two groups of 25 and 29. You’re not going to foil our plans this time, sneaky #4237.

Field work plays many important roles in a researcher’s career. It provides the context for an entire project and an escape from tedious lab work. Although I am studying soil science, there is always the larger ecological framework that underpins agricultural systems, that I enjoy taking the time to ponder and incorporate into my questions while being in the field. Field work allows me to see my theories about grazing and soil properties play out, sometimes shifted from expectations due to some unexpected animal behavior. Now, I reflect fondly on my field season and bring up a picture of a happy sheep grazing whenever I need to bring a smile to my face. And I hope the sheep (Venti, #7008, #4237, and all the rest!) also remember their action-packed few days, getting out of the UCD sheep barn and on to tasty cover crop. 

Sequoia Williams is a first year student in the Soils and Biogeochemistry graduate group in Dr. Amelie Gaudin’s Agroecology lab. Sequoia grew up raising sheep in 4-H and is happy to have sheep back in her life. She studies integrated crop-livestock systems and their impacts on soil health and soil carbon and nitrogen pools. 

References:

1.        Méndez, V. E., Bacon, C. M. & Cohen, R. Agroecology as a Transdisciplinary, Participatory and Action-Oriented Approach. Agroecol. Sustain. Food Syst. 37, 3–18 (2013).

2.        Russelle, M. P., Entz, M. H. & Franzluebbers, A. J. Reconsidering integrated crop-livestock systems in North America. Agron. J. 99, 325–334 (2007).

3.        2019 California Processing Tomato Report. USDA, National Agricultural Statistics Service (2019).