Effect of undegradable protein concentration in the post-weaning diet on body growth and reproductive development of Assaf rams

Abstract

Twenty-four growing Assaf lambs, divided into four groups of six animals, were used to study the effect of the undegradable protein content of the post-weaning diet on feed intake, body growth and reproductive development. From week 1 to week 21, the four groups were fed ad libitum as follows: group LL was given barley straw and low protein concentrate (LP); group HH was given barley straw and high protein concentrate (HP); group LH was given barley straw and LP concentrate from week 1 to 11 (period 1) and barley straw and HP concentrate from week 12 to 21 (period 2); group HL was given barley straw and HP concentrate in period 1 and barley straw and LP concentrate in period 2. From week 22 to week 26 (period 3), all animals received the same amount of hay and LP concentrate. Barley straw intake was not significantly (P > 0.05) affected by dietary treatments. In the 1st period, average concentrate intake and live body weight gain (LWG) were greater in lambs fed HP than LP supplement. In the 2nd period, concentrate intake was not significantly (P > 0.05) affected by type of supplement, but LWG was greater for lambs fed HP than LP supplement. Scrotal circumference in week 11 was significantly (P < 0.05) lower in lambs fed LP supplement than in lambs fed HP supplement. No significant differences (P > 0.05) due to dietary treatments were observed on scrotal circumference in weeks 21 and 25. Dietary treatments had no significant (P > 0.05) effect on either circulating concentration of testosterone or ejaculate characteristics. In conclusion, results from this study suggest that supplementing diets with undegradable protein enhanced performance throughout the breeding period and accelerated testis growth. Nevertheless, final testis size, pattern of circulating testosterone and sperm output were unaffected by dietary treatments.

Introduction

The study of the reproductive function in different ruminant species has provided evidence for the effects of nutrition during the growing period on the development of the hypothalamic-pituitary gonadal axis and hence on the onset of puberty. It has been reported that in ruminants low planes of nutrition during the prepuberal period delay testicular growth and the onset of puberty by inhibiting the development of a mature reproductive endocrine system [1]. Severe underfeeding can even permanently impair sperm output [2].

On the other hand, it has long been established that rams reared on higher levels of feeding grow faster and attain puberty at younger ages than rams on low planes of nutrition [3], [4]. Nevertheless, it is noteworthy that there is some controversy about the effects of additional nutritive supply, above maintenance requirements, during the prepuberal period on testicular development and semen characteristics. Thus, it has been reported that the reproductive potential of young males may also be impaired by overfeeding. Coulter and Kozub [5] observed a detrimental effect of high energy intake on 2-year-old Hereford bulls, as measured by epididymal sperm reserves and sperm motility and morphology. Morrow et al. [6] evaluated the effects of low and high energy rations on the growth and reproductive development of Angus and Simmental bulls and found that the low energy group showed higher values of fertility and libido. Likewise, it has been suggested in gilts that high lean growth rates may delay the onset of puberty [7]. It has also been shown in rats that an excess of certain amino acids can result in testis degeneration with tubules lacking spermatids and with subnormal Leydig cells [8].

In contrast to the above-mentioned results, several studies did not reveal any effect of the level of intake during the pre- and post-puberal period of young bulls and rams on reproductive traits, such as testicular size, semen quality or serving capacity, upon reaching adulthood [1], [9], [10]. Other studies, however, suggest a positive effect on the level of intake during the breeding period on reproductive potential of the animals. In relation to this, it has recently been reported that Sertoli cell number is greater in ram lambs growing fast than in those with moderate growth rates [11]. Germ cell number is limited by the number of Sertoli cells, which do not normally proliferate after puberty [12].

In relation to the controversial effects above-mentioned, it is necessary to point out that the complexity of the rumen ecosystem has limited progress in understanding the effect of dietary energy to protein interaction on body growth and sexual development. In ruminants, the protein flowing to the duodenum is the sum of the amount of dietary protein that escapes ruminal degradation and the quantity of microbial protein synthesised in the rumen, which depends on both the fermentable energy and the degradable protein content of the diet [13]. Therefore, the amount and quality of protein arriving at the small intestine cannot be adequately predicted from the digestible crude protein or crude protein content of the diet. Moreover, it is currently well known that even when optimising microbial protein production, undegradable protein supply is also needed to meet protein requirements of animals with high body growth rates [13].

Furthermore, in addition to the difficulties in designing adequate protein supplementation strategies for ruminant diets, the effects of the protein to energy ratio of the diet on body growth and development can vary depending on the breed or the phase of the growing period [14]. To our knowledge, information about this subject in the Assaf breed is scarce.

The objective of the present study was to test the hypothesis that undegradable protein supply, leading to differences in growth rate in pre- and post-puberal male Assaf rams, affects testicular growth, plasma testosterone concentration and sperm production and quality.