Nutritional strategies to improve the reproductive performance of beef females

Graduation date: 2015In the first set of studies, 2 experiments evaluated the influence of supplement composition on ruminal forage disappearance, performance, and physiological responses of Angus × Hereford cattle consuming a low-quality, cool-season forage (8.7 % CP and 57 % TDN). In Exp. 1, 6 rumen-fistulated steers housed in individual pens were assigned to an incomplete 3 x 2 Latin square design containing 2 periods of 11 d each and the following treatments: 1) supplementation with soybean meal (PROT), 2) supplementation with a mixture of cracked corn, soybean meal, and urea (68:22:10 ratio, DM basis; ENER), or 3) no supplementation (CON). Steers were offered meadow foxtail (Alopecurus pratensis L.) hay for ad libitum consumption. Treatments were provided daily at 0.50 and 0.54 % of shrunk BW/steer for PROT and ENER, respectively, to ensure that PROT and ENER intakes were isocaloric and isonitrogenous. No treatment effects were detected on rumen disappearance parameters of forage DM (P ≥ 0.33) and NDF (P ≥ 0.66). In Exp. 2, 35 pregnant heifers were ranked by initial BW on d -7 of the study, allocated into 12 feedlot pens (4 pens/treatment), and assigned to the same treatments and forage intake regimen as in Exp. 1 for 19 d. Treatments were fed once daily at 1.77 and 1.92 kg of DM/heifer for PROT and ENER, respectively, to achieve the\ud same treatment intake as % of initial BW used in Exp. 1 (0.50 and 0.54 % for PROT and\ud ENER, respectively). No treatment effects (P = 0.17) were detected on forage DMI. Total\ud DMI was greater (P < 0.01) for PROT and ENER compared with CON, and similar\ud between PROT and ENER (P = 0.36). Accordingly, ADG was greater (P = 0.01) for\ud PROT compared with CON, tended to be greater for ENER compared with CON (P =\ud 0.08), and was similar between ENER and PROT (P = 0.28). Heifers receiving PROT\ud and ENER had greater mean concentrations of plasma glucose (P = 0.03), insulin (P ≤\ud 0.09), IGF-I (P ≤ 0.04), and progesterone (P₄; P = 0.01) compared to CON, whereas\ud ENER and PROT had similar concentrations of these variables (P ≥ 0.15). A treatment ×\ud hour interaction was detected (P < 0.01) for plasma urea N (PUN), given that PUN\ud concentrations increased after supplementation for ENER and PROT (time effect, P <\ud 0.01), but did not change for CON (time effect; P = 0.62). In conclusion, beef cattle\ud consuming low-quality cool-season forages had similar ruminal forage disappearance and\ud intake, performance, and physiological status if offered supplements based on soybean\ud meal or corn at approximately 0.5 % of BW (DM basis).\ud The following experiment evaluated the influence of supplement composition on\ud performance, reproductive, and metabolic responses of Angus × Hereford heifers\ud consuming a low-quality cool-season forage (8.7 % CP and 57 % TDN). Sixty heifers\ud (initial age = 226 ± 3 d) were allocated into 15 drylot pens (4 heifers/pen; 5\ud pens/treatment), and assigned to the same treatments as reported above. Heifers were\ud offered meadow foxtail (Alopecurus pratensis L.) hay for ad libitum consumption during\ud the experiment (d -10 to 160). Beginning on d 0, PROT and ENER were provided daily at\ud a rate of 1.30 and 1.40 kg of DM/heifer to ensure that PROT and ENER intakes were\ud isocaloric and isonitrogenous. Hay and total DMI were recorded for 5 consecutive days\ud during each month of the experiment. Blood was collected every 10 d for analysis of\ud plasma P₄ to evaluate puberty attainment. Blood samples collected on d -10, 60, 120, and\ud 150 were also analyzed for PUN, glucose, insulin, IGF-I, NEFA, and leptin. Liver\ud samples were collected on d 100 from 2 heifers/pen, and analyzed for mRNA expression\ud of genes associated with nutritional metabolism. No treatment effect was detected (P =\ud 0.33) on forage DMI. Total DMI, ADG, mean concentrations of glucose, insulin, and\ud IGF-I, as well as hepatic mRNA expression of IGF-I and IGFBP-3 were greater (P ≤\ud 0.02) for PROT and ENER compared with CON, and similar between PROT and ENER\ud (P ≥ 0.13). Mean PUN concentrations were also greater (P < 0.01) for PROT and ENER\ud compared with CON, whereas PROT heifers had greater (P < 0.01) PUN compared with\ud ENER. Plasma leptin concentrations were similar between ENER and PROT (P ≥ 0.19),\ud and greater (P ≤ 0.03) for ENER and PROT compared with CON on d 120 and 150\ud (treatment × day interaction; P = 0.03). Hepatic mRNA expression of mitochondrial\ud phosphoenolpyruvate carboxykinase was greater (P = 0.05) in PROT compared with\ud CON and ENER, and similar between CON and ENER (P = 0.98). The proportion of\ud heifers pubertal on d 160 was greater (P < 0.01) in ENER compared with PROT and\ud CON, and similar between PROT and CON (P = 0.38). In conclusion, beef heifers\ud consuming a low-quality cool-season forage had a similar increase in DMI, growth, and\ud overall metabolic status if offered supplements based on soybean meal or corn at 0.5 % of\ud BW.\ud The last experiment was designed to determine if frequency of protein\ud supplementation impacts physiological responses associated with reproduction in beef\ud cows. Fourteen non-pregnant, non-lactating beef cows were ranked by age and BW, and\ud allocated to 3 groups. Groups were assigned to a 3 × 3 Latin square design, containing 3\ud periods of 21 d and the following treatments: 1) soybean meal (SB) supplementation\ud daily (D), 2) SB supplementation 3 times/wk (3WK), and 3) SB supplementation\ud once/wk (1WK). Within each period, cows were assigned to an estrus synchronization\ud protocol; 100 μg of GnRH + controlled internal drug release (CIDR) containing 1.38 g of\ud P4 on d 1, 25 mg of PGF₂α on d 8, and CIDR removal + 100 μg of GnRH on d 11. Grassseed\ud straw was offered for ad libitum consumption. Soybean meal was individually\ud supplemented at a daily rate of 1 kg/cow (as-fed basis). Moreover, 3WK were\ud supplemented on d 0, 2, 4, 7, 9, 11, 14, 16, and 18, whereas 1WK were supplemented on\ud d 4, 11, and 18. Blood samples were collected from 0 (prior to) to 72 h after\ud supplementation on d 11 and 18, and analyzed for PUN. Samples collected from 0 to 12 h\ud were also analyzed for plasma glucose, insulin, and P4 (d 18 only). Uterine flushing fluid\ud was collected concurrently with blood sampling at 28 h for pH evaluation. Liver biopsies\ud were performed concurrently with blood sampling at 0, 4, and 28 h, and analyzed for\ud mRNA expression of carbamoylphosphate synthetase I (CPS-I; h 28), and CYP2C19 and\ud CYP3A4 (h 0 and 4 on d 18). Plasma urea-N concentrations were greater (P < 0.01) for\ud 1WK vs. 3WK from 20 to 72 h, and greater (P < 0.01) for 1WK vs. D from 16 to 48 h\ud and at 72 h after supplementation (treatment × hour interaction; P < 0.01). Moreover,\ud PUN concentrations peaked at 28 h after supplementation for 3WK and 1WK (P < 0.01),\ud and were greater (P < 0.01) at this time for 1WK vs. 3WK and D and for 3WK vs. D.\ud Expression of CPS-I was greater (P < 0.01) for 1WK vs. D and 3WK. Uterine flushing\ud pH tended (P ≤ 0.10) to be greater for 1WK vs. 3WK and D. No treatment effects were\ud detected (P ≥ 0.15) on expression of CYP2C19 and CYP3A4, plasma glucose and P4\ud concentrations, whereas plasma insulin concentrations were greater (P ≤ 0.03) in D and\ud 3WK vs. 1WK. Hence, decreasing frequency of protein supplementation did not reduce\ud uterine flushing pH or plasma P₄ concentrations, which are known to impact\ud reproduction in beef cows.\ud In summary for all the experiments presented herein: (1) pregnant and developing\ud replacement beef heifers consuming a low-quality, cool-season forage equally utilize and\ud benefit, in terms of growth and metabolic parameters, from supplements based on protein\ud or energy ingredients provided at approximately 0.5 % of heifer BW/d, (2) energetic\ud supplementation at approximately 0.5 % BW/d did not impair forage disappearance\ud parameters in rumen-fistulated steers, and (3) decreasing soybean meal supplementation\ud frequency to once a week did not increase uterine pH, plasma P₄, and expression of\ud hepatic enzymes associated with steroid catabolism in ruminants