The FF Plus expressed shock at the cruelty of the murder and said the government should help ensure the safety of farming communities.
"The government and minister of police are slow to act against farm attackers, and the rural safety plan still exists only in theory," FF+ MP Pieter Groenewald said in a statement.
The police's Brigadier Thulani Ngubane earlier said that three suspects had entered Van der Merwe's house at about 20:00.
"They held him at gunpoint while they ransacked the house for money," he said on Sunday.
"They shot him in the chest, the back, and in the head, then used [the farmer's] bakkie to drag him for about 1.2km before the bakkie rolled."
According to Ngubane his body was still tied to the Toyota LDV when police arrived on the scene on the Ottosdal-Delareyville road.
One of the alleged murderers was arrested there and led police to his two accomplices. It was suspected that some cattle may have been stolen in the attack.
Ngubane said although the brutality of the attack might suggest an "ulterior motive", possibly related to "previous issues with the farmer", he could not confirm whether the suspects were known to Van der Merwe, who lived alone.
"This will all form part of the investigation and it is too early to speculate at the moment."
Cases of murder, house robbery, theft of motor vehicle, stock theft and illegal firearm possession had been opened against the three arrested men. They would appear in court on Tuesday.