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The Influence of Seasonal Differences on the Growth of Navajo Lambs

United States Department of Agriculture¹

Abstract

Groups of Navajo lambs raised at the Southwestern Range and Sheep Breeding Laboratory, Fort Wingate, New Mexico, have been weighed at intervals of four weeks from birth to weaning age (20 weeks) each year from 1938 to 1941, inclusive. The lambs were all born during the two middle weeks of May and had a range of one week on either side of the mean age.

Highly significant differences between the mean weights of the four years were found at all ages except birth. Analyses of variance showed that whenever significant differences between the mean weights occurred, the variance between the years was responsible for most of the variations in weights. Analyses carried out on two select groups of lambs yielded similar results.

The importance of uncontrollable environmental factors was shown by the agreement between the lamb weights and the precipitation received for the same period. It was pointed out that range forage was dependent upon the amount and time of precipitation, and that the amount of forage largely determined the growth made by the lambs. During 1938 to 1940 drouth conditions prevailed, while the amount of moisture received in 1941 was considerably above normal. The lambs weighed in 1941 were not only heavier but they were also thriftier than those weighed in the previous years. These differences were attributed to the favorable season.

¹ This study was conducted at the Southwestern Range and Sheep Breeding Laboratory, Fort Wingate, N. Mex., under authority of the Bankhead-Jones Act, in cooperation with the Bureau of Indian Affairs, United States Department of the Interior.