What a coherent stream means
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The mechanism
As liquid leaves, the headspace must be replaced by air. If the neck is narrow and the liquid fills it, air and liquid fight for the same opening and the flow breaks into pulses.
Why pulses are the problem the rule addresses
Each pulse throws liquid wider than a steady stream, and a concentrated pesticide splashing out of a measuring jug is the exposure the regulation is trying to prevent.
What fixes it
A wider neck, a shoulder that lets air track back along the top of the bottle, or a vented closure. All three are decisions taken before the container exists.
Viscosity moves the threshold
A thin fertilizer solution glugs at a neck size where a thicker concentrate pours cleanly, and a product that thickens in the cold will behave differently in a shed in February.
Test with the formula, cold
Water tells you about water. Pour the actual product at the temperature a user will meet it, into the vessel they will pour it into, and watch whether the stream breaks.
Drip is a separate requirement
Minimising drip is about the lip and the closure rather than the neck. A bottle that pours beautifully and then drips down its own front panel fails half the requirement.
Which argues for a measuring closure
A dosing chamber or a spouted closure meters and cuts off cleanly. The shelf carries 18 spouted closures, and for a concentrate they do more work than any other part of the package.
Fill level changes the behaviour
A full bottle glugs differently from a half-empty one, because the headspace that has to be replaced is a different volume. Test at the start, the middle and the last pour; the last one is usually the worst.
And the user is wearing gloves
Whatever the test says on a bench, the real pour happens outdoors, often in gloves, into a measuring jug balanced on something. A shape that needs a careful grip to pour cleanly will not get one.