Let us look at the graph of tree rings growth of different clusters provided by Yu.P. Demakov.

Ref.: Yu. P. Demakov. Factors of annual tree growth dynamics. http://csfm.marstu.net/presentation/Demakov.pdf

There is an analogy. Is there a relationship between the cereal yield growth in the world and the Earth's rotational velocity?

Yes, there is such a relationship. But it mostly manifested itself in the 90s. At the beginning of the XXI century it became less pronounced. Note: correlation between the tree growth and the Earth's rotational velocity is negative in Russia, whereas for the cereals worldwide it is positive.

Will these dependencies remain regarding the intensity of the northern and southern solar magnetic fields?

On the graph above the cereal yield in Arab countries and in the USA is time-coordinated with the change of intensity of the solar magnetic field.

Let us enlarge the part of the graph, which is closer in time to the inversion of the magnetic field on the Sun in 2013.

In the USA, cereal yield is higher than in Arab countries when the northern magnetic pole is positive. With the sign change of the solar magnetic pole the graph of cereal yield growth in these countries diverges. For the cereals that grow in the USA the period when the northern magnetic pole has a positive charge is more favorable. Whereas for the cereals that grow in Arab countries the period when the northern magnetic pole has a negative charge is more favorable. But this is also connected with the Earth's rotational velocity.

Magnetic fields of the Sun and the Earth, as well as the velocity of its rotation, are all interconnected.

There is the graph to support this:

First, the Earth's magnetic field changes, after that – its rotational velocity. Although, at the scale of hundreds of years, this shift is not very noticeable.

Growth of plants is statistically related to the change of the Earth's magnetic field and with the change of its rotational velocity. Take into account that in different regions of the Earth the change dynamics of the magnetic field varies. Therefore, the shift to obtain a mapping between the graphs of the Earth's rotational velocity and the magnetic field intensity might be different in time.

The difference in cereal growth in the United States and Arab countries is higher at the greater difference in the intensity of the southern and northern solar magnetic fields.

Intensity of the solar magnetic pole is related to intensity of the Earth's magnetic pole. This relationship can be understood by analyzing the graph of Ukrainian scientists (V.D. Rusov, et al., 2011).

There is a relationship, a correspondence, but it is shifted in time.

What relationship is there between the cereal yield in the world and the intensity of the Earth's magnetic field? To learn this, let us take the data of space monitoring of the magnetic field intensity in various regions of the Earth.

Swarm reveals Earth’s changing magnetism, ESA, 19 June 2014, http://www.esa.int/Our_Activities/Observing_the_Earth/Swarm/Swarm_reveals_Earth_s_changing_magnetism

We will also present a graph of the change in the Earth's rotational velocity on the day when the data of its magnetic field were obtained.

It can be seen that the intensity of the magnetic field over North America is higher than over Arab countries. Then maybe this is the reason why we have a difference in cereal growth rates?

We will recheck this by comparing cereal growth in the world (matches cereal growth in the USA) with the growth in Argentina and Brazil where the magnetic field intensity is much lower.