This sentence is a traditional proverb in rural China, which expresses the relationship between seasons and weather.
In spring, cattle are the main agricultural tools, which are used for plowing and sowing. If cows don't go to the fields for farming in spring, the fields will be deserted and farmers will miss the best time for planting.
And "The Dragon King flies to the sky in summer" is actually a metaphor. In China culture, the Dragon King is the god in charge of rain.
In the ancient concept, if you don't farm in spring, there will be no rain in summer, and if the Dragon King doesn't come down to rain, it will lead to crops not growing and the harvest will be reduced.