When it comes to cutesy exchange-traded fund ticker symbols, perhaps the agriculture-related sector tops the list. The first of the group to come out with a colorful symbol was the Market Vectors Agribusiness ETF (MOO), which commenced in August 2007.

Three more ETFs with distinctive ticker symbols hit then scene earlier this year. In March, the IQ Global Agribusiness Small Cap ETF (CROP) was launched. In May and June, Global X Funds launched the Global X Fertilizers/Potash (SOIL) and Global X Farming ETF (BARN) funds, respectively.

Makes you wonder what's next: GROW? SEED?

In comparison, the ticker symbol of the sector's oldest fund, the PowerShares DB Agriculture ETF (DBA), seems rather boring

But catchy exchange listings aside, the proliferation of ag-related ETFs illustrate the industry's growing trend to slice-and-dice the available investing world into ever-refined niches. Beyond the aforementioned funds, the ag space also includes the PowerShares Global Agriculture Potrfolio fund (PAGG).

And on the exchange-traded note side, there's also the iPath Dow Jones-UBS Livestock Subindex Total Return ETN (COW), which is based on an index of livestock contracts in lean hogs and live cattle traded on U.S. exchanges. The ETN recently was down roughly 40% since it started trading in October 2007.

Based on demographic trends, it appears the agriculture sector is golden for the long haul. The planet needs to feed its ever-growing population, and the rising middle class in developing nations means more demand for animal-based protein--which requires tons of grain to produce. And there's also a big demand for biofuels.

Investors who want to play the space through ETFs need to look under the tractor hood to see how these funds differ and whether they're a good fit for their portfolio. And they need to realize that despite the ag-sector's favorable long-term outlook, it can be a volatile place to invest.

DBA
The DBA fund was launched in January 2007 and is based on the DBIQ Diversified Agriculture Index Excess Return index. This index holds futures contracts in feeder cattle, cocoa, coffee, corn, cotton, lean hogs, live cattle, soybeans, sugar, wheat, and Kansas wheat. By holding some of the most liquid and widely-traded agricultural commodities, the index aims to reflect the performance of the broader ag sector. As of the end of March, the fund's top weightings were in corn, soybeans, sugar and live cattle (12.5% each).

Among ag-based ETFs, DBA is the only one based on futures contracts (the rest are equity-based). One of the banes of commodity futures contracts is contango, which occurs when a far-future delivery price costs more than a nearer-future delivery price. Investors lose money when ETFs roll expiring contracts into more expensive future contracts.

DBA tries to mitigate contango by looking at the next 13 months worth of contracts and calculating the implied roll yield on all 13 to choose the most optimal period. That was easier back in the days when the fund followed just four commodities. In 2009, the fund expanded its index from four to 11 commodities.

Currently, five of the commodities that underpin the fund use optimized yield roll to combat contango, but the other six don't because those commodities aren't liquid enough to employ that strategy, according to PowerShares.

DBA's share price peaked at nearly $42 in July 2008, then plummeted later that year when the entire global economy went down the drain. After a prolonged period in the $23 to $27 range, the fund made a big jump staring last summer when ag commodities zoomed thanks to two factors: favorable supply-and-demand fundamentals, and the general spike in commodity prices after the Federal Reserve's second round of quantitative easing. The fund topped $35 in March, but reversed course to around $32 as of late June when ag-based commodities--along with most other commodities--tumbled as global markets retreated.

DBA sported a recent market cap of nearly $3.2 billion, making it the second-largest ag-based ETF.

MOO
This fund tracks the DAXglobal Agribusiness index comprised of companies that get at least 50% of their revenue from the ag business. This includes chemical, equipment and operations companies, along with livestock operations and ethanol/biodiesel producers. As of the end of March, the index's top five holdings were Deere & Co., Potash Corp. of Saskatchewan, Monsanto Co., The Mosaic Co. and Syngenta AG. This is mainly a large-cap ETF (about 85% holdings).

At a recent market cap of $5.3 billion, it's the largest of the ag ETFs.

The price charts for both DBA and MOO track similar patterns--they hit all-time peaks around mid-2008, followed by precipitous drops during the downturn, and then rapid ascents that began last summer. Of late, both funds took it on the chin in concert with the overall decline in ag and other commodities.

Since its inception, DBA had returned roughly 32%. MOO has gained roughly 28% since it began trading. But according to Yahoo Finance, MOO has outperformed DBA by about 28% versus 21% since MOO's inception eight months after DBA's debut.

PowerShares says MOO's outperformance relative to DBA is due to MOO's higher equity beta and not necessarily because of DBA's diminished ability to fight contango that resulted after it expanded its underlying index to include commodities that don't employ the optimized roll strategy.

As for expenses, MOO sports a 0.56% ratio versus DBA's 0.75%.

CROP
The small-cap focus of this fund differentiates it from other equity-based ag ETFs. The fund tracks the IQ Global Agribusiness Small Cap index that encompasses crop production; livestock operations; agriculture supply, logistics, machinery, and chemicals; and biofuels.

Its recent top holdings were Tractor Supply, Viterra Inc. (a Canadian-based feed, milling and fertilizer company), Smithfield Foods, Nippon Meat Packers, and Nutreco N.V., (a Dutch-based animal and fish feed maker).

The fund's expense ratio is 0.75%. After its March launch, the fund had an initial pop before dipping below its first-day price as of late June.


SOIL
If agriculture is a niche investment sector, then SOIL qualifies as a micro niche sector. The fund's underlying Solactive Global Fertilizers/Potash index pretty much sums up the fund's focus. The index focuses on fertilizer producers necessary to boost crop yields to feed a growing, hungry world. The underlying investment thesis is that demand is outstripping supply, which sounds reasonable in the long run but didn't seem to matter when the fund debuted on May 26 at a time when fertilizer stocks were on a losing streak. The fund is currently below it's first-day trading price.

SOIL's top five recent holdings--all of which produce fertilizers--were Yara International (Norway), Uralkali-Spon GDR (Russia), CF Industries (U.S.), Sociedad Quimica Y Minera (Chile) and Intrepid Potash Inc. (U.S.).

Its annual expense ratio is 0.69%.

BARN
As the new kid on the block, BARN started trading on June 1. The fund is based on the Solactive Global Farming index comprised of 50 global companies engaged in agriproduct and livestock operations, as well as the manufacturing or distribution of farming products such as tractors and irrigation systems.

BARN's basic approach is similar to that of MOO, with the latter holding more concentrated positions in the agri-chemical space, in U.S. companies, and in its top ten holdings.

BARN's annual expenses are slightly more than MOO, at 0.68%. The fund's top five holdings as of late June were Tate & Lyle (a British food ingredients company), Wilmar International Ltd. (a Singapore-based agribusiness group), Kubota Corp., Agco Corp. (a U.S.-based ag equipment maker), and Monsanto.

PAGG
PAGG's share price nosedived shortly after its debut in September 2008. By late June 2011, the fund could boast having a cumulative gain of about 20% during its short life. The fund is based on the Nasdaq OMX Global Agriculture index that measures the largest and most liquid global agriculture and farming-related companies. It's recent top five holdings were Syngenta, Monsanto, Potash Corp. of Saskatchewan, Mosaic and Wilmar.

The expense ratio is 0.75%.

The Upshot
When sifting through ag ETFs, investors have a choice between futures-based (DBA) or equity-based (all the rest), small-cap (CROP) versus larger cap (the other equity-based offerings), or thinly-traded offerings versus more liquid funds (DBA and MOO).

"The first thing you need to do is determine what kind of exposure you're looking for, and that's not just with ag but with other commodity funds," says Morningstar ETF analyst Abraham Bailin.

On the futures side with DBA, he says, the fund tracks its underlying index very closely but not necessarily the actual commodities because of the way it uses futures contracts to mitigate contango risk.

MOO is the big gorilla in the space, but its huge weighting in ag-chem companies might be a turn-off to some investors. People who want to diversify their ag holdings could turn to the small-cap CROP fund.

And with ETFs, liquidity is frequently a consideration. For example, Bailin notes that over the past three months, MOO's average daily trading volume was roughly 2 million shares versus about 400,000 shares for PAGG.

Bailin says he doesn't have a problem with any of these funds, but in the end he'd opt for the two biggest ETFs in the space. "There are only two that matter, and that's DBA and MOO," he says.