Managing mature beef bulls on divergent planes of nutrition alters scrotal circumference and concentrations of hormones and metabolites

Fifteen mature beef bulls (4 and 5 years old; BW = 1,816 ± 38.3 lb) were used in each of two years to evaluate effects of divergent planes of nutrition on concentrations of hormones and metabolites. In Year 1, bulls were ranked by BW and randomly assigned to one of two treatments for a 112-d evaluation period; 1) managed on a positive plane of nutrition (POS), or 2) managed on a negative plane of nutrition (NEG). In Year 2 bulls were assigned to the opposite treatment they received in Year 1 (i.e. POS in Year 1 were assigned to NEG in Year 2, and vice versa). Bulls were fed a common diet with deliveries into Insentec feeders adjusted biweekly to achieve targeted weight loss or gain (~12.5% of original BW). Blood samples were collected on d 0, 56, and 112 and analyzed for concentrations of amino acids (AA) in Year 1 and for glucose, non-esterified fatty acids (NEFA), testosterone (T), triiodothyronine (T3), thyroxine (T4), and insulin-like growth factor-1 (IGF-1) in Year 1 and Year 2. By design, bull BW was influenced by a treatment × day interaction (P < 0.001), with POS bulls being heavier (P < 0.01) than NEG bulls by d 28. Over the course of the experiment POS bulls gained 2.74 ± 0.10 lb/d while NEG lost 2.35 ± 0.10 lb/d. Body condition score and scrotal circumference were also impacted by treatment × day interactions (P < 0.001), both starting similar among treatments, then greater for POS than NEG thereafter. To achieve targeted weight divergence POS bulls (30.4 ± 0.99 lb/d) ate more (P < 0.0001) than NEG bulls (11.2 ± 0.99 lb/d). Concentrations of glucose, NEFA, T3, T4, and IGF-1 were influenced by treatment × day interactions (P < 0.001). Concentrations of glucose, T3, T4, and IGF-1 were greater (P < 0.01) for POS bulls on d 112 compared with NEG bulls on the same days. Concentrations of NEFA, however, were greater (P < 0.001) for NEG than POS on d 56 and 112. Total amino acids present in serum were impacted by a treatment × day interaction (P < 0.001), with POS bulls having more (P ≤ 0.001) AA present in serum than NEG bulls on d 56 and 112. Our model resulted in altered body composition and profiles of hormones and metabolites which could have effects on testicular tissue and semen at functional, morphological, and molecular levels. Further investigation into the fertility and offspring outcomes resulting from our model of divergent bull nutrition are ongoing.

Dramatic and dynamic changes in body weight and plane of nutrition occur within a year in the life of breeding bulls. Factors contributing to weight loss in mature bulls may be due to work load and nutritional management. A survey of producers revealed that stocking rates varied from 4 cows per bull up to 80 cows per bull (Dahlen and Stoltenow, 2015), and bulls can experience dramatic weight loss; between 100 and 400 lb (Walker et al., 2009; Hersom and Thrift, 2008). Bulls losing weight during the breeding season must subsequently be managed to regain lost weight (Barth 2013).

Producer decisions determine the point at which bulls begin losing and gaining weight relative to the breeding season. In some scenarios, bulls begin losing weight only at the beginning of breeding season, and are then managed to gain weight thereafter, reaching targeted optimal weight just before the subsequent breeding season. Bulls in this scenario are in a positive energy balance over the time course of spermatogenesis. In other scenarios bulls may start losing weight before the breeding season. Perhaps these bulls experienced a recent change in environment and diet after purchase, or perhaps they were managed to gain weight over winter and needed to be cut back to get into “breeding shape” or placed on pastures to graze ahead of the breeding season. In either instance, these bulls would be on a negative plane of nutrition leading up to the breeding season. When we evaluate the two respective scenarios together we see a major and common divergence in plane of nutrition leading up to the breeding season.

Spermatogenesis is a continual process that takes roughly 61 d for sperm development, followed by up to 14 d residence in the epididymis prior to ejaculation (Senger, 2012). The net result is that sperm used to inseminate a cow today likely began the process of development up to 75 d before breeding. Thus, divergence in plane of nutrition likely exposes sperm to different hormonal profiles and metabolic substrates during the time of spermatogenesis, residence in the epididymis, and upon combination with seminal plasma at ejaculation. The consequences of these differing environments remain underexplored. Therefore, our objectives were to evaluate divergent planes of nutrition on body composition and concentrations of hormones and metabolites.

All procedures were approved by the North Dakota State Institution for Animal Care and Use Committee.

Fifteen mature beef bulls (4 and 5 years old; BW = 1,816 ± 38.3 lb) were used in each of two years to evaluate effects of divergent planes of nutrition on body composition and concentrations of hormones and metabolites. In Year 1 bulls were ranked by BW and randomly assigned to one of two treatments for a 112-d evaluation period; 1) managed on a positive plane of nutrition (POS), or 2) managed on a negative plane of nutrition (NEG). In Year 2 bulls were assigned to the opposite treatment they received in Year 1 (i.e. POS in Year 1 were assigned to NEG in Year 2, and vice versa). In each year bulls were fed a common diet containing corn silage, triticale hay, cracked corn, dried distiller’s grains plus solubles, and a vitamin/mineral premix (Table 1). Diets were placed in Insentec feeders with deliveries adjusted based on biweekly body weight to achieve targeted weight loss or gain (~12.5% of original BW). Scrotal circumference and body condition score were determined every 28 days.

Blood samples were collected on d 0, 56, and 112 from the tail vein. Samples were allowed to clot for 30 minutes and centrifuged at 1,500 × g at 4 C for 20 minutes. Serum samples were separated and stored in plastic vials at -20°C until further analysis. Samples were analyzed using the Synergy H1 Microplate Reader (Biotek, Winooski, VT) with the Infinity Glucose Hexokinase Kit (Thermo Scientific, Waltham, MA) and NEFA-C Kit (WAKO Chemicals, Inc., Richmond, VA). Serum samples were analyzed for concentrations of testosterone (T), triiodothyronine (T3), thyroxine (T4), and insulin-like growth factor-1 (IGF-1) by competitive chemiluminescent immunoassay using the Immulite 1000 (Siemens, Los Angeles, Calif.). Concentrations of total amino acids were determined in serum samples from Year 1 only using the ACQUITY Ultra-Performance Liquid Chromatography System (Waters Corporation, Milford, Mass.).

Data were analyzed using the MIXED procedures of SAS for effects of treatment, collection day, year, and their respective interactions with bull as the experimental unit. Differences were considered significant at a P ≤ 0.05.

By design, BW of bulls on the respective treatments diverged over the course of the experiment. Bulls on the POS treatment tended (P = 0.08) to be heavier than NEG bulls by d 14 of the experiment, with differences significant by d 28 (P < 0.01; Figure 1, Panel A) and continuing through the evaluation period. Over the course of the experiment POS bulls gained 2.74 ± 0.10 lb/d while NEG lost 2.35 ± 0.10 lb/d. To achieve targeted weight divergence POS bulls (30.4 ± 0.99 lb/d) ate more (P < 0.000) than NEG bulls (11.2 ± 0.99 lb/d).

Figure 1. Body weight (Panel A), body condition score (Panel B), and scrotal circumference (Panel C) of beef bulls managed on divergent planes of nutrition. Bulls on POS were targeted to gain 12.5% of BW over 112 d, whereas bulls on NEG were targeted to lose 12.5% of initial BW over 112 d. # within day means tend to differ P ≤ 0.10; † within day means differ P = ≤ 0.05; * within day means differ P ≤ 0.01.

Body condition score was also impacted by a treatment × day interaction (P < 0.001), with BCS starting similar among treatments, then being greater (P < 0.01) for POS than NEG from d 28 to 112 (Figure 1, Panel B). By the end of the evaluation period there was a 2.3 BCS unit difference between treatments. Additionally, scrotal circumference was impacted by a treatment × day interaction (P < 0.001), with no difference present at the beginning of the experiment, but divergence between treatments beginning on d 56 and continuing through the end of the evaluation period (Figure 1, Panel C).

Concentrations of glucose, NEFA, T3, T4, and IGF-1 were influenced by treatment × time interactions (P < 0.001; Table 2).

1- Collection day. Bulls on POS were targeted to gain 12.5% of BW over 112 d, whereas bulls on NEG were targeted to lose 12.5% of initial BW over 112 d.
x,y,z - Means within row lacking common superscript differ (P ≤ 0.05).

Concentrations of glucose, T3, T4, and IGF-1 were greater (P < 0.01) for POS bulls on d 112 compared with NEG bulls on the same days. However, concentrations of NEFA were greater (P < 0.001) for NEG than POS on d 56 and 112. Concentrations of testosterone evaluated from a single blood sample before feeding were not impacted by the treatment × day interaction (P = 0.44) or by the main effect of treatment (P = 0.40). Concentrations of testosterone did increase (P < 0.001) through the evaluation period as days lengthened and the traditional breeding season approached. As testosterone is released episodically from Leydig cells in respond to pulses of LH from the pituitary, future work should include serial sampling or GnRH challenges to more precisely evaluate the impact of bull nutrition on concentrations of testosterone.

Total amino acids present in serum were impacted by a treatment × day interaction (P < 0.001), with POS bulls having more (P ≤ 0.001) AA present in serum than NEG bulls on d 56 and 112 of the evaluation period (Figure 2).

Figure 2. Total amino acids present in serum of bulls managed on divergent planes of nutrition. Bulls on POS were targeted to gain 12.5% of BW over 112 d, whereas bulls on NEG were targeted to lose 12.5% of initial BW over 112 d. * within day means differ P ≤ 0.01.

Under the common production scenarios evaluated in this experiment, fluctuations in body weight and plane of nutrition of breeding bulls lead to changes in blood hormone and metabolite profiles. Increased hormone and metabolite concentrations in POS bulls were a product of enhanced plane of nutrition, and elevated NEFA in NEG bulls was indicative of bulls mobilizing body reserves and a source of energy. The observed alterations in blood profiles likely resulted in alterations of nutrients available for developing sperm. Whether and how these different blood nutrient profiles influenced cellular function in the testis and in sperm produced should be further evaluated. Specific efforts being undertaken with our model of divergent planes of nutrition include evaluating novel measures of fertility via flow cytometry, evaluating the mRNA and miRNA of resultant sperm, and evaluating in vitro fertility and embryo development, with the ultimate goal of evaluating offspring outcomes.

The authors would like to thank the students at the NDSU Beef Cattle Research Complex for their help in completion of the project. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. The USDA is an equal opportunity provider and employer.

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Hersom, M and T. Thrift. 2008. Nutritional Management of Bulls. University of Florida AN211. Available at: AN21100.pdf (ufl.edu)

Senger, P. L. 2012. Pathways to Pregnancy and Parturition. 3rd ed. Current Conceptions Inc., Redmon, OR.

Walker, J., G. Perry, R. Daly, and K. Olson. 2009 Bull management and nutrition. Range Beef Cow Symposium XXI.