海外で目指すビジネスのねらいについて、下村さんは、「僕らは『世界の農業の頭脳』になりたい。データを集めることで、例えば、 _地球温暖化の原因 **と**
されるメタンガスをあまり出さない牛を_どうしたら作ることができるかも提案できる」と夢を語ってくれました。

Is と working here as WITH or IF or what? Personally I think is WITH, but I am
not so sure.

My breaking down of the sentence: *地球温暖化の原因 **と**
されるメタンガスをあまり出さない牛をどうしたら作ることができるかも提案できる

earth warming's cause (WITH or IF or WHAT)done methane gas を not-much-emit cow
を what things make が able may propose be able to.
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particles
How is と working here?
This is a quotative use of と. 下村さん is explaining his aspiration for creating
an overseas business.

The main sentence is

> 海外で目指すビジネスのねらいについて、下村さんは、夢を語ってくれました。

The nature of his dream is expressed in the と clause

>
> 僕らは『世界の農業の頭脳』になりたい。データを集めることで、例えば、地球温暖化の原因とされるメタンガスをあまり出さない牛をどうしたら作ることができるかも提案できる

Here’s a partial rendering

> Regarding his aspiration for an overseas business, Mr Shitamura told us
> about his dream of how they would like to become world experts in
> agriculture....

There is a second と in the phrase 原因とされる but in context, that's

> 地球温暖化の原因とされるメタンガス

which is a phrase where .

> 地球温暖化の原因とされる

is a relative clause. This clause says

> Methane, which is the source of global warming,...

Other than these two instances, there is no third instance of the particle と.
saying

Since from your comments it seems you wrestling with how these parts all fit
together here is how to render the portion about their ambition:

> For example, from our collected data, we will be able to propose how we
> might create cows that don’t produce much methane which is a source of
> global warming.
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> 〔地球温暖化の原因 **と** される〕メタンガス

As the other poster says, 地球温暖化の原因とされる is a relative clause that modifies
メタンガス.

Now how about thinking of it this way...

Its non-relative version is: メタンガス **が** 地球温暖化の原因 **とされる**

「XXがYYとされる」 -- "XX is regarded as YY" "XX is considered to be YY".

So this literally means "Methane **is regarded as** the source of global
warming".

And its non-passive version can be: （我々が）メタン **を** 地球温暖化の原因 **とする**

「（人が）XXをYYとする」 -- "(Someone) regard XX as YY".

which literally means "(We) **regard** methane **as** the source of global
warming".

So the phrase in your sentence literally means:

> "Methane, which is regarded as the source of global warming"

For this grammar point, @Naruto's answer in this thread might help:

[Did I parse and interprete this sentence
correctly?](https://japanese.stackexchange.com/q/56395/9831)

* * *

How about parsing the sentence this way:

> 〔〈（地球温暖化の原因とされる）メタンガスをあまり出さない〉牛をどうしたら作ることができるか*〕も提案できる

_Lit._ "We/I will be able to propose 〔how we/they can make cows 〈which don't
emit much methane （which is considered as the source of global warming）〉〕.

*「どうしたら/どうすれば～～できるか」「どうしたら/どうすれば + potential」 means "How we can do~~" "What we should do to~~".
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