The incident occurred Tuesday evening at Bhuyanpara range in the park, officials said Wednesday adding although the forest guards launched an operation against the poachers immediately they failed to retrieve the horn.
"We heard three rounds of firing from inside the park yesterday (Tuesday) evening and launched an operation immediately. Later, we found a female rhino with its horn removed," said a senior official.
In another incident that took place in a different location inside the park, forest guards killed a poacher Tuesday.
"We recovered bullets of AK series rifle from the body of the rhino," the officials said, adding this indicated the poachers were armed with sophisticated weapons.
This is the sixth rhino to be killed by poachers inside the park this year.
Officials said the killed rhino was brought from the Pabitora Wildlife Sanctuary of the state last year as part of the ongoing rhino translocation programme and it gave birth to a calf in Manas last year.
The United Nations Educational, Scientific and Cultural Organisation declared Manas National Park a World Heritage site in December 1985. However, in 1992, the park was declared a "World Heritage Site in Danger" due to heavy poaching and terrorist activities.
The park also lost all its one-horned rhinos during the period and the Assam government started the rhino re-introduction programme in 2006. In 2011, the park was removed from the list of "World Heritage Site in Danger".
Poachers have also killed at least 23 one-horned rhinos in the state's Kaziranga National Park this year.
The rampant killings have threatened the ambitious Indian Rhino Vision 2020 project which aims at increasing the animal's population in Assam to 3,000 by 2020 and also ensure rhinos are distributed at least over seven protected areas.
--IANS (Posted on 30-10-2013)