Oocyte quality and viability in Nguni and Hereford cows exposed to a high protein diet

Abstract

Protein is a fundamental part of nutrition in all animals but ruminants have the ability to metabolise and utilise non-protein nitrogen for this use. This allows for an economical alternative source of protein that does not compete with human resources. Previous evidence suggests that feeding high levels of protein to support increased demands for growth and production may have a negative impact on reproduction, since protein is rapidly converted to ammonia and further metabolised to urea which are both toxic to the reproductive system. Investigations within the South African climate indicated that the Nguni cow maintained higher blood urea nitrogen (BUN) levels during periods of drought than other cattle breeds. The Nguni breed is well adapted to extreme situations but as its popularity grows and the breed is promoted for emerging farmers it becomes essential to know if it possesses adaptations which could predispose it to reproductive failure if supplemented in the same manner as other commercial breeds. In this prospective experimental study, 22 multiparous cows of two breeds, the Hereford and the Nguni, were block randomised into a cross over design whereby all animals were fed a total mixed ration (TMR). The test group was provided with an increasing rumen degradable protein content in the form of feed grade urea. The control group was maintained on a urea free TMR. In the test group, the urea inclusion rate was increased weekly by 50 g to allow for rumen microflora adaptation and prevent acute urea toxicity. They reached a maximum inclusion rate of 200 g urea per cow per day. All animals had serum collected and oocytes aspirated by transvaginal ultrasound guided oocyte pick up (OPU) were counted and graded twice weekly. Following this, all usable oocytes were pooled per breed and treatment, and were subject to in vitro maturation, fertilisation and culture in order to assess the effect of elevated protein on oocyte competence and viability. Oocyte quality was defined as the number of oocytes with cumulus cells (Grade 1 3 oocytes) harvested per OPU, and oocyte viability was defined as the number of oocytes that cleaved by day 2 or that reached at least the morula stage by day 7 in the in vitro embryo system. Multivariable analyses were performed on BUN, antral follicle count (AFC) and number of grade 1 3 oocytes harvested per OPU session, and on the oocyte viability outcomes in the vitro embryo system. Nguni cows receiving 150 g dietary urea per day had lower mean BUN levels than Herefords (17.5 and 19.3 mg/dL respectively, P = 0.02). However dietary urea inclusion level, serum albumin level and sampling day were the only independent predictors of serum BUN. Nguni cows had lower mean antral follicle count (AFC) and number of oocytes harvested than Herefords (9.1 and 3.1, and 11.7 and 4.3 respectively, P < 0.01). The vet performing the aspiration, within day sampling order and BUN level >20 mg/dL were independently associated with the number of grade 1-3 oocytes after adjusting for AFC and the random effect of the individual animal. Increasing weighted mean serum albumin and decreasing weighted mean beta-hydroxybutyric acid (BHBA) were independently associated with the number of oocytes that cleaved (day 2) or that reached the morula stage (day 7) (P = 0.01 and P = 0.08 respectively). It was concluded that increasing dietary urea, low BCS and increasing serum albumin, but not breed, were associated with increasing BUN levels in cows. It was further concluded that BUN >20 mg/dL adversely affected the occurrence of cumulus cells around oocytes and that serum albumin and BHBA were, but neither dietary urea level nor breed was independently associated with viability of bovine oocytes in this study. Although the data were inconclusive about breed differences in protein metabolism, this study suggests that serum albumin has the potential to be used as predictor of the interaction between protein and energy metabolism and its effect on oocyte development.