Effect of supplemental nutrient source on heifer growth and reproductive performance, and on utilization of corn silage-based diets by beef steers.

Two experiments were conducted to determine effects of oilseeds or soybean hulls on growth and reproductive performance of heifers and utilization of corn silage diets by growing beef cattle. In Exp. 1, 96 beef heifers (249 kg of BW) were used in a randomized complete block design. Treatments were as follows: 1) corn and soybean meal (CON) at 56% of the DMI; 2) whole linted cottonseed at 15% of the DMI (COT); 3) whole raw soybeans at 15% of the DMI (SB); or 4) pelleted soyhulls at 30% of the DMI (SH). Diets were formulated to be isonitrogenous (13.8% CP) and fed to achieve target weights equal to 65% of expected mature BW at the time of AI. Estrus was synchronized and heifers were inseminated by AI in response to detected estrus. Because the energy value for SH was underestimated, cumulative ADG for SH (1.03 kg/d) was greater (P < or = 0.03) than for CON (0.89 kg/d), COT (0.87 kg/d), or SB (0.86 kg/d). Treatment did not affect (P > 0.10) the proportion of pubertal heifers at the beginning of the breeding season: CON (60%), COT (53%), SB (69%), SH (71%), or first-service conception rates: CON (37%); COT (38%); SB (57%); SH (42%). In Exp. 2, crossbred steers (387 kg) were used in a 6 x 6 Latin square design to evaluate the effects of supplemental nutrient source on utilization of corn silage diets. Treatments included diets used in Exp. 1, plus a negative control (soybean meal at 10% of the DMI; SIL) and whole raw soybeans at 25% of the DMI (SB25). Diets were formulated to be isonitrogenous (13.8% CP) except SB25 (17% CP), and were fed twice daily at 1.8 x NEm. Oilseed inclusion decreased (P < 0.10) acetate:propionate ratios and (P < 0.10) apparent ruminal OM and ruminal and total tract NDF digestibilities. The CON and SH diets had the greatest (P < 0.10) total-tract OM digestibilities. Microbial efficiencies were greatest (P < 0.10), and long chain fatty acid flow to the duodenum increased (P < 0.10) with oilseeds. Biohydrogenation averaged 90.4% and increased slightly (P < 0.10) when oilseeds were added to the diet. Adding oilseeds or soybean hulls to corn silage-based diets did not affect reproductive performance of heifers. Although oilseed additions increased total fatty acid flow to the duodenum, a high degree of biohydrogenation occurred, greatly increasing C18:0, with only marginal increases in unsaturated fatty acid flow. Depending on diet and feeding conditions, inclusion of whole oilseeds may not be an effective means of increasing linoleic acid supply for ruminant animals.

Characterization of season and sampling method effects on measurement of forage quality in fescue-based pastures.

Information on seasonal changes and effects of sampling methods on the measurement of forage quality is limited for fescue-based pastures. Eight continuously grazed, 0.76-ha, fescue-based pastures were used to compare forage type, method of collection, and seasonal effects on forage quality in a repeated-measures, split-plot design. Four pastures were interseeded with red clover in March 2000. Masticate (M; from four ruminally cannulated steers) and hand--clipped (C) samples were collected every 28 d from April to October 2000. Interseeding red clover did not affect (P > 0.10) OM, CP, NDF, and ADF concentrations or CP degradability. Sampling method and season interacted (P < 0.03) for OM, CP, NDF, and ADF concentrations. Concentrations of OM averaged 5 percentage units more (P < 0.01) in C than in M in all months and were more variable with M than with C. Samples clipped between April and September averaged 5.5 percentage units greater NDF (P < 0.01), 3.0 percentage units greater ADF (P < 0.01), and 4.5 percentage units less CP (P < 0.01) than masticate samples obtained during the same time period. Fiber and CP concentrations did not differ (P > 0.10) between C and M samples obtained in October. Differences in CP degradability estimates (using Streptomyces griseus protease) between the two sample types were greater in late-season samples than in samples obtained from April to June. When S. griseus protein degradability estimates were compared with in situ estimates for masticate samples, no differences (P > 0.10) were detected early in the season (April to June). However, the S. griseus procedure overestimated in situ values (P < 0.01) by an average of 3 percentage units in samples obtained between July and October. Differences in composition of C and M samples were substantial until late season, when opportunities for selective grazing were minimal. Small differences between S. griseus and in situ estimates of CP degradability indicate that the S. griseus procedure can yield useful CP degradability estimates for fescue-based pasture samples. Although it might be possible to apply correction values to clipped samples to estimate CP and fiber concentrations of diets selected by grazing cattle, inconsistent relationships preclude this approach for estimates of CP degradability.