Can feeding pomegranate peel to cows reduce greenhouse gas emissions?
Is climate change real? A couple of years ago, when I first came to Norway, winter was a fun season. In winter, much of Norway usually transformed into a snow-lad paradise. This meant I could go skiing nearly every day in the snow-covered field right next to the campus. However, it is oddly enough that there has not been much snow falling in the last few years. It has got me thinking whether this climate phenomenon is a result of global warming due to the increase of greenhouse gas (GHG) emissions. As an animal nutritionist, I have been thinking about what we can do to reduce the GHG emissions from large animals such as dairy cows. Would feeding pomegranate peel to cows reduce greenhouse gas emissions and valorize a food industry by-product?
Fruit peel
Millions of tons of fruit peel are produced annually worldwide and disposed of, which is problematic due to high costs (approx. 10 million US dollar spent annually in the US on the disposal of peel solely from apple) and environmental pollution of landfills. Livestock number will increase with the increasing human population, and alternative feeds are needed which do not compete with human food production. The pomegranate (Punica granatum L.) is an important cash crop native from Iran to the Himalayas in northern India and has been cultivated since ancient times throughout the entire Mediterranean region of Asia, Africa, and Europe. Today, important pomegranate producers are Iran (120,000 t/a), Turkey, India, China (1,600,000 t/a; on 110,000 ha) and the USA (>1000 farms; on 13,300 ha; mainly California). During the industrial processing of pomegranate to juice, the peel, pulp, and seeds of the pomegranate fruit arise as by-products and are forming the peel.
Functional ruminant feed – pomegranate peel
View of pomegranate peel pieces used in the experiment (Photo: Alfred Galke GmbH).
Dairy cows are very efficient animals in the way they transform plant fiber into meat and dairy products for human consumption. Methane is produced as a by-product, which is more than 25 times as potent as carbon dioxide at trapping heat in the atmosphere. In addition, dairy cows excrete nitrogen in the form of urea which is easily volatile and prone to be emitted from the manure, among others as nitrous oxide, another GHG. The GHG emissions should therefore be limited as much as possible. The strategic use of functional feeds like pomegranate peel may have a double benefit (1) to lower greenhouse gas emissions and (2) to promote the utilization of food industry by-products in livestock nutrition.
The pomegranate peel has a relatively high nutritive value. Furthermore, it is a valuable source of plant secondary compounds, such as polyphenols, especially tannins, and saponins, and thus a potential functional feed ingredient in ruminant diets for improving the redox and health status and protein utilization. In our previous in vitro study, the peel of pomegranate was most promising among nine fruit and vegetable peels tested, in reducing ruminal methane yield. Therefore, we aimed to investigate the effect of pomegranate peel on methane emission, productivity, and antioxidant status of dairy cows.
The experiment
Pomegranate peel was purchased from Alfred Galke GmbH (Bad Grund, Germany). Upon its arrival, we realized that the peel was of a very hard texture, and we were a bit concerned that the cows might not like to eat it and avoid it even in the mixed diet. We got already mentally prepared to grind the 700 kg of pomegranate peel to obtain a powder that we can mix into the dairy cows' diet to trick them into eating it. Therefore, we were very happy when we heard constant crunchy noises after mixing the pomegranate peel pieces into the diet, which confirmed that the animals were indeed eating these pieces. Luckily, we could start the animal experiment as planned. One animal even seemed to enjoy the peel more than the rest of the diet because the top-dressed peel was gone before we even had a chance to mix it by hand with the rest of the diet. All animals received the same basal mixed ration (maize silage and grass silage, alfalfa, concentrate, straw, and hay) that was supplemented with either 5% or 10% of the peel or remained unsupplemented as a control.
Following an adaptation to the diet for 14 days, we collected samples of milk as well as separate feces and urine collection of the individual cows, so that we could check the quality of milk and the digestibility of the diet. To measure their methane emissions, the animals were transferred to open-circuit respiration chambers for the measurement in the last two days of each period.
A cow in a respiration chamber (Photo: Puchun Niu).
Working with the cows was a bittersweet experience. We needed to collect urine samples with a home-made urinal for 24 h for 7 days in a row. It was a bit surprising to see that there was not any loss of the urine samples, maybe a few drops, in the first few days of collection, until one or two urinals started leaking. We had to spend some time fixing the urinals because we were afraid of losing too much of the samples. What we learned from that is to either make enough urinals to replace the ones malfunctioning or to improve the quality of the urinals if we were to do experiments like this.
A cow made time for a picture while eating (Photo: Puchun Niu).
Another important aspect of the experiment was to take care of the animals, including feeding, cleaning, and brushing, which made me feel like having them as pet animals. I was happy to see their eagerness for the fresh feed when they saw me holding a spading fork approaching the feed in front of them. I enjoyed seeing them eating calmly and sometimes raising their heads while eating, which gave me an opportunity to take a picture (Figure 3). I also got embarrassed by myself when I tried to talk to them, in a way that I thought they could understand, without receiving any response. Like humans, cattle are social creatures with complex behaviors, such as grooming behavior which generally involves one cow licking another around the head and neck. It was often seen during the experiment that cows adjacent to one another tended to groom the individual that had previously groomed them. Cows frequently grooming each other is a positive sign that means that those cows get along.
Preliminary results
Our preliminary results suggest that feed palatability, milk production, and milk gross composition were not affected by the supplementation of pomegranate peel. Likewise, and against our expectations, the methane emissions did not differ between the dietary treatments. It seems the amount of pomegranate peel added was probably too low. Nevertheless, the blood and milk urea concentration, and urinary nitrogen excretion were lower with the 10% pomegranate peel diet compared to the control treatment. In conclusion, the inclusion of up to 10% pomegranate peel did not compromise palatability and performance of cows and alleviated metabolic and environmental nitrogen load but did not mitigate enteric methane emissions.
Still to come
One group of bioactive components present in pomegranate are the tannins. Tannins are known to be able to modulate rumen processes and have positive antioxidant effects, but they might also mediate toxic effects to the animal. By analyzing metabolic markers in the blood of the animals, we want to evaluate if the rather low dosages of dietary pomegranate peel might have positive antioxidant effects or may have induced already some adverse effects on the animals. Moreover, the analysis of the milk fatty acid profile as an important determinant of the nutritional quality of the milk for human consumption is still ongoing.
ABOUT THE AUTHOR
Puchun Niu is a PhD student, studying nutrition of ruminant livestock at the Norwegian University of Life Sciences. He studied veterinary science for his bachelor and feed manufacturing technology for his master, which led him to develop a particular interest in ruminant nutrition. He is currently working on prediction, microbial heritability, and mitigation of methane emissions from dairy cows. He works to predict methane emissions through prediction models for national inventory and studies the relationship between methane emission and rumen microbiota.
I would like to acknowledge World Food System Center for funding the project. I would like to acknowledge Prof. Michael Kreuzer for hosting my stay at ETH Zurich, and Dr. Katrin Giller and Prof. Angela Schwarm for supervising me during the experiment. I would also like to thank them for inspiring me to write the blog post and editing it. It has been a great pleasure working with them on the project.