Digestion during continuous culture fermentation when replacing perennial ryegrass with barley and steam-flaked corn.

The objective of this study was to quantify the optimal inclusion rate of grain required to maximize nutrient digestion of a diet based on highly digestible pasture. It was hypothesized that maximum digestion would occur at a rate of grain inclusion that resulted in a culture pH of 6.0, reflecting the pH below which fiber digestion would be expected to be compromised. Four dual-flow continuous culture fermenters were used to establish the effects on digestion of replacing freeze-dried, highly digestible ryegrass with 0, 15, 30, and 45% of dry matter as 60% barley, 35% steam-flaked corn, and 5% molasses mix. The respective composite diets were fed twice daily to mimic intake patterns observed in dairy cows offered supplements during milking and offered half their daily allowance of pasture after each milking. Digesta samples were collected during the last 3 d of each of four 9-d experimental periods. Average daily culture pH decreased linearly as proportion of cereal grain in the diet increased, with average daily pH ranging from 6.29 to 5.74. Concentrations of neutral detergent fiber and total fatty acids decreased linearly with increasing proportion of cereal grain in the diet. Digestion of organic matter (OM) was maximized at an interpolated value of 24% grain inclusion and culture pH of 6.0, but the difference in the OM digestibility over the range of grain treatments from 0 to 45% was small (3 percentage units) despite pH changes over a range of 6.3 to 5.7. The relatively small change in OM digestibility was explained by reduced fiber and crude protein digestibilities being balanced by an increased digestion of nonstructural carbohydrate. Although different relationships between ruminal pH and digestibility appear to exist when cows are fed pasture alone compared with a total mixed ration, when starch supplements are included in pasture diets, the relationships associated with feeding a total mixed ration may then be more likely to apply.

Effects of strain of Holstein-Friesian and concentrate supplementation on the fatty acid composition of milk fat of dairy cows grazing pasture in early lactation.

The effect of a grain-based concentrate supplement on fatty acid (FA) intake and concentration of milk FA in early lactation was investigated in grazing dairy cows that differed in their country of origin and in their estimated breeding value for milk yield. It was hypothesized that Holstein-Friesian cows of North American (NA) origin would produce milk lower in milk fat than those of New Zealand (NZ) origin, and that the difference would be associated with lower de novo synthesis of FA. In comparison, increasing the intake of concentrates should have the same effect on the FA composition of the milk from both strains. Fifty-four cows were randomly assigned in a factorial arrangement to treatments including 3 amounts of concentrate daily [0, 3, and 6 kg of dry matter (DM)/cow] and the 2 strains. The barley/steam-flaked corn concentrate contained 3.5% DM FA, with C18:2, C16:0, and C18:1 contributing 48, 18, and 16% of the total FA. The pasture consumed by the cows contained 4.6% DM FA with C18:3, C16:0, and C18:1 contributing 51, 10, and 10% of the FA, respectively. Pasture DM intake decreased linearly with supplementation, but total DM intake was not different between concentrate or strain treatments, averaging 16.2 kg of DM/cow, with cows consuming 720 g of total FA/d. Cows of the NA strain had lesser concentrations of milk fat compared with NZ cows (3.58 vs. 3.95%). Milk fat from the NA cows had lesser concentrations of C6:0, C8:0, C10:0, C12:0, C14:0, and C16:0, and greater concentrations of cis-9 C18:1, C18:2, and cis-9, trans-11 C18:2, than NZ cows. These changes indicated that in milk from NA cows had a lesser concentration of de novo synthesized FA and a greater concentration of FA of dietary origin. Milk fat concentration was not affected by concentrate supplementation. Increasing concentrate intake resulted in linear increases in the concentrations of C10:0, C12:0, C14:0, and C18:2 FA in milk fat, and a linear decrease in the concentration of C4:0 FA. The combination of NA cows fed pasture alone resulted in a FA composition of milk that was potentially most beneficial from a human health perspective; however, this would need to be balanced against other aspects of the productivity of these animals.

Variation in plasma concentrations of insulin-like growth factor-I in pasture-fed Holstein cows.

The objective of this 5-wk study was to determine dietary effects on plasma concentrations of insulin-like growth factor-I (IGF-I), as well as milk production and milk components in pasture-fed dairy cows. Thirty-two Holstein cows 4 to 5 wk postpartum were randomly assigned to 4 dietary subgroups. Feed was provided twice daily ad libitum at 0900 and 1600 h composed of fresh-cut pasture, meadow hay, and pelleted cereal grain to achieve differing levels of DMI and ME density (LL: 16.6 kg of DMI and 174 MJ of ME; HL: 17.3 kg of DMI and 181.1 MJ of ME; LH: 15.4 kg of DMI and 183.1 MJ of ME; HH: 17.9 kg of DMI and 213.3 MJ of ME, with the first letter indicating DMI and the second ME, and with H indicating high and L indicating low, respectively). The first day cows were placed on their diets was designated d 0. Concentrations of IGF-I were measured in frozen-thawed samples of plasma using a verified ELISA. Dietary treatment had affected plasma concentrations of IGF-I by d 7 with cows on high ME diets having greater IGF-I concentrations at d 14 (83.7 vs. 45.6 ng/mL) than cows on the low ME diets. The level of DMI had less effect on plasma concentrations of IGF-I at d 14 (72.2 vs. 57.1 ng/mL). Dietary treatment effects on these concentrations had stabilized by d 21. Day-to-day variation in mean plasma concentrations of IGF-I within each dietary treatment was low during an intensive period of daily sampling for 14 d (from d 22 to 35). Within-cow day-to-day variation was also low compared with that among cows within the same dietary group and was associated with a high repeatability in the day-to-day concentration of IGF-I in individual cows. Intraclass correlation coefficients for IGF-I ranged from 0.56 (+/- 0.14) to 0.88 (+/- 0.06) with a combined (pooled) value for the 4 subgroups of 0.77 (+/- 0.05). The ME and DMI effects (H vs. L) at d 35 were 79.3 vs. 41.4 and 62.0 vs. 55.7 ng/mL, respectively. Although the ME and DMI differences also affected milk yield and compositional parameters, the effects were not as proportionately great as those measured for IGF-I. Altering the ME or DMI components of the pasture-based diets produced changes in plasma IGF-I concentrations that did not become stabilized for 3 wk, but were then highly repeatable for individual cows within each dietary group. Both observations have relevance to interpreting data related to plasma concentrations of IGF-I in lactating Holstein cows.

Measuring concentrations of insulin-like growth factor-I with an enzyme-linked immunosorbent antibody assay in plasma samples from Holstein cows.

This study describes the use of a commercial nonextraction ELISA to quantify total concentrations of insulin-like growth factor-I (IGF-I) in plasma samples from Holstein cows fed pasture-based diets varying in dry matter and metabolizable energy density. The assays were conducted using the protocols provided by the manufacturer. The ELISA was verified for linearity, accuracy in measuring IGF-I from spiked plasma samples, and precision involving variation within and between assays. Validation also involved comparing results of the ELISA against an established RIA after defatted plasma samples were subjected to acid gel HPLC to dissociate and separate IGF and IGF binding protein complexes. Validation results had low coefficients of variation (CV; intraassay CV of less than 6% and interassay CV of less than 8%) and a high recovery percentage of IGF-I (79%) from samples spiked with unbound IGF-I. The coefficients of determination between the ELISA and the RIA reference assay were 0.90 in 2 separate assays. Associations between the RIA and the ELISA were higher, and the limits of agreement at 95% confidence intervals were narrower compared with those between the RIA and a modified ELISA procedure in which IGF binding protein complexes were extracted using HPLC. The samples were obtained from cows sampled over a period of 5 wk. These results demonstrated that the DSL-10-2800 nonextraction IGF-I ELISA was acceptably specific and sufficiently sensitive to be used to measure the extent and patterns of change in the plasma concentrations of IGF-I in samples from lactating Holstein cows.

Diurnal variation in ruminal pH on the digestibility of highly digestible perennial ryegrass during continuous culture fermentation.

Dairy cows grazing high-digestibility pastures exhibit pronounced diurnal variation in ruminal pH, with pH being below values considered optimal for digestion. Using a dual-flow continuous culture system, the hypothesis that minimizing diurnal variation in pH would improve digestion of pasture when pH was low, but not at a higher pH, was tested. Four treatments were imposed, with pH either allowed to exhibit normal diurnal variation around an average pH of 6.1 or 5.6, or maintained at constant pH. Digesta samples were collected during the last 3 d of each of four, 9-d experimental periods. A constant pH at 5.6 compared with a constant pH of 6.1 reduced the digestibility of organic matter (OM), neutral detergent (NDF), and acid detergent fiber (ADF) by 7, 14, and 21%, respectively. When pH was allowed to vary (averaging 5.6), digestion of OM, NDF, and ADF were reduced by 15,30, and 36%, respectively, compared with pH varying at 6.1. There was little difference in digestion parameters when pH was either constant or varied with an average pH of 6.1. However, when average pH was 5.6, maintaining a constant pH significantly increased digestion of OM, NDF, and ADF by 5, 25, and 24% compared with a pH that exhibited normal diurnal variation. These in vitro results show that gains in digestibility and potential milk production can be made by minimizing diurnal variation in ruminal pH, but only when ruminal pH is low (5.6). However, larger gains in productivity can be achieved by increasing average daily ruminal pH from 5.6 to 6.1.

Nutrient supply to dairy cows from processed white lupines.

The objective of this study was to compare the DVE/OEB system (DVE = truly absorbed protein in the small intestine; OEB = degraded protein balance (DPB) in Dutch) and the NRC-2001 model in the prediction of supply of protein to dairy cows from processed white lupines (Lupinus albus L.). Comparisons were made in terms of (1) ruminally synthesized microbial CP, (2) truly absorbed protein in the small intestine, and (3) degraded protein balance. In addition, the systematic investigation of roasting of the white lupines at various temperatures (110, 130, or 150 degrees C) and times (15, 30 or 45 min) on manipulation of digestive behaviour and the potential nutrient supply to dairy cows were also carried out, to obtain information on best processing conditions as intestinal protein sources (to achieve target values for potential high net absorbable protein in the small intestine while holding any N loss in the rumen to a low level). The results showed that the predicted values from the DVE/OEB system and the NRC-2001 model had significant correlations with high R ( > 0.83) values. However, using the DVE/OEB system, the overall average microbial protein supply based on available energy was 11% higher and the truly absorbed protein in the small intestine was 7% higher than that predicted by the NRC-2001 model. The difference was also found in the prediction of the degraded protein balances (DPB), which was 8% higher based on data from the NRC-2001 model. These differences are due to considerably different factors used in calculations in the two models, although both are based on similar principles. This indicates that a further refinement is needed for a modern protein evaluation and prediction system. In addition, this study showed that the roasting at higher temperature and time was effective in shifting protein degradation from rumen to intestines and it increased the DVE or MP values without reaching the negative degraded protein balance. The processing at 15 degrees C for 30 or 45 min might be the best treatments for white lupine due to its higher DVE and MP values and the very low DPB values.

Effects of dietary lipid type on muscle fatty acid composition, carcass leanness, and meat toughness in lambs.

Isonitrogenous amounts of two protein sources differing in rumen degradation rate and in lipid composition were fed to sheep with or without a rapidly fermentable cereal grain. The effects on intake, carcass leanness, and muscle fatty acid (FA) composition were examined. Thirty-eight crossbred wether lambs (9 mo, 35 to 48 kg) were allocated by stratified randomization to six treatment groups: 1) basal diet of alfalfa hay:oat hay (20:80) ad libitum = basal; 2) basal + lupin (358 g DM/d) = lupin; 3) basal + fish meal (168 g DM/d) = fish meal; 4) basal + barley (358 g DM/d) = barley; 5) basal + barley + lupin (179 + 179 g DM/d) = barley/lupin; or 6) basal + barley + fish meal (179 + 84 g DM/d) = barley/ fish meal. Lambs were fed individually. Dietary treatments were imposed for 8 wk, and the supplements were offered at 2-d intervals. Daily feed intake and weekly BW of lambs were recorded. At the end of the feeding period lambs were slaughtered after an overnight fast. Hot carcass weight (HCW) and fat depth (GR; total fat and muscle tissue depth at 12th rib, 110 mm from midline) were recorded. At 24 h postmortem samples of longissimus thoracis (LT) and longissimus lumborum (LL) muscles were taken from chilled (4 degrees C) carcasses for the assessment of FA composition and meat tenderness, respectively. Lambs fed lupin or fish meal with or without barley had heavier slaughter weights (P < 0.004) and HCW (P < 0.001) than lambs fed basal or barley when initial BW was included as a covariate. The lupin diet also resulted in heavier carcasses (P < 0.05) than the fish meal or barley/fish meal diets. With GR as an indicator, fish meal and barley/ fish meal diets produced leaner carcasses (P < 0.01) than lupin and barley/lupin lambs. Long-chain n-3 FA content [20:5n-3 (P < 0.001), 22:5n-3 (P < 0.003), and 22:6n-3 (P < 0.001)] in the LT muscle were substantially higher with the fish meal and barley/fish meal diets, whereas muscle total n-6 FA was increased (P < 0.003) by lupin and barley/lupin compared with all other diets. Thus, increased muscle long-chain n-3 FA content occurred without an increase in fatness measured as GR, whereas increased muscle n-6 FA content was associated with an increase in carcass fatness. Under these circumstances, a reduction in carcass fatness had no effect on meat tenderness measured as Warner-Bratzler shear force.

Effects of pre and antenatal elevated and chronic oMt1a-oGH transgene expression on adipose deposition and linear bone growth in mice.

Exposing growing oMtla-oGH transgenic mice with the regulatable metallothionein promotor to elevated growth hormone (GH) for three weeks after weaning enhances bone length and adipocyte differentiation. The objective of the present study was to investigate the consequences of highly elevated GH exposure during fetal and early postnatal growth periods on the mature phenotype. Transgene expression, hence elevated GH, was achieved in fetuses and neonates by providing 25 mM ZnSO4 to the drinking water of the dams. Wildtype and oMtla-oGH male and female mice were a) never exposed to the transgene stimulus, b) exposed from birth to 21 d of age, c) exposed through gestation until 21 d of age, d) exposed only through gestation, or e) exposed only during the first 7 d postpartum. At 84 d of age when mature body size was reached, ulna and humerus lengths, and body, liver gonadal fat pad, mesenteric fat pad, and cleaned gastrointestinal (GI) tract weights were recorded. Bone lengths were also determined in a subset of mice at 22 d of age. While early exposure to the elevated GH increased ulna and humerus length at 22 d of age, the early GH levels failed to produce significant changes in adipose content or bone lengths at maturity. However, chronic exposure to slightly elevated GH, as seen in the transgenics never induced to express the transgenic GH, depressed liver and GI weights and increased adipose depot weights and humerus lengths across both sexes. These results suggest that certain tissues in the body, while capable of responding to GH during early developmental periods, are not fully entrained to sustain that growth response once the GH stimulus is withdrawn. Further, the preadipocyte pool appears unable to respond to GH early in development. Finally, the tissues examined exhibited a differential response to the GH suggesting that different tissues possess distinct response thresholds.

Effect of dietary modification of muscle long-chain n-3 fatty acid on plasma insulin and lipid metabolites, carcass traits, and fat deposition in lambs.

In a previous study we showed that feeding fish meal significantly increased muscle long chain n-3 fatty acids (FA) and hot carcass weight. In this study we compared the effect of fish meal and fish oil on increasing muscle long-chain FA. We also investigated whether the increase in carcass weight was due to the effect of dietary enrichment of muscle long-chain n-3 FA on muscle membrane phospholipids and(or) to rumen by-pass protein provided by fish meal. Forty crossbred ([Merino x Border Leicester] x Poll Dorset) wether lambs between 26 and 33 kg BW were randomly assigned to one of five treatments: 1) basal diet of oaten:lucerne chaff (Basal); 2) Basal + fish meal (9% DM) = FM; 3) Basal + fish oil (1.5% DM) with protected sunflower meal (9% DM ) = FOSMP; 4) Basal + fish oil (1.5% DM) = FO; or 5) Basal + protected sunflower meal (10.5% DM) = SMP. Daily intake of ME (9.60 - 10.5 MJ ME/d) and CP (150 to 168 g/d) in all treatments was kept similar by varying the ratio of oaten:lucerne chaff and by feeding the animals at 90% ad libitum intake. Blood samples were collected at the start of the experiment and on the day (d 42) prior to slaughter. Lambs were then slaughtered at a commercial abattoir. At 24 h postmortem carcass traits were measured and longissimus thoracis muscle taken for analysis of FA of phospholipid and triglyceride fractions. Lambs fed FO and FOSMP showed a marked increase in muscle longchain n-3 FA (P < 0.001) and a reduction in magnitude of the rise in insulin concentration (P < 0.001) after feeding compared with lambs fed Basal and SMP diets. Lambs in FM had a moderate increase (P < 0.001) in muscle long-chain n-3 FA content. Compared with Basal diet, both plasma total cholesterol (P < 0.02) and high-density lipoprotein cholesterol (P < 0.001) levels were greater in SMP and less in FO and FOSMP treatments. The i.m. fat content was reduced (P < 0.05) in FM and FO treatments, but carcass weight was increased only with fish meal (P < 0.03). Adding SMP to FO produced muscle with an intermediate level of i.m. fat, whereas muscle long-chain n-3 FA, i.m. fat, and insulin concentration were unchanged with SMP treatment. These results indicate that an increase in carcass weight in FM may be due to the supply of ruminally undegraded protein. They also suggest that fish oil along with fish meal can increase long-chain n-3 FA content in phospholipid of muscle membrane. This may be associated with reduced i.m. fat content and altered insulin action and lipoprotein metabolism.

Effect of diets containing n-3 fatty acids on muscle long-chain n-3 fatty acid content in lambs fed low- and medium-quality roughage diets.

In two experiments, each with 32 cross-bred ([Merino x Border Leicester] x Poll Dorset) wether lambs (26 to 33 kg weight range), animals were randomly assigned to one of four treatments. A mixture of lucerne chaff:oaten chaff was used as a basal diet, offered in different ratios. Animals were allowed to consume on a free-access basis in Exp. 1 or 90% of ad libitum intake in Exp. 2 in order to provide a low- (6.5 MJ ME/d) and medium- (9.5 MJ ME/d) quality basal diet, respectively. Isoenergetic amounts of lipid supplements, fish meal (80 g DM), canola meal (84 g DM), and soy meal (75 g DM) were tested in Exp. 1. In Exp. 2, fish meal (9% DM), unprotected rapeseed (7% DM), and protected canola seed (6% DM) were fed as supplements. At the end of 53-d (Exp. 1) or 46-d (Exp. 2) experimental periods, lambs were slaughtered at a commercial abattoir and at 24 h postmortem longissimus thoracis (LT) muscle was collected for the analysis of fatty acid (FA) composition of structural phospholipid and storage triglyceride fractions. Fish meal diet increased LT muscle long-chain n-3 FA content by 27% (P < 0.02) in Exp. 1 and 30% (P < 0.001) in Exp. 2 compared with lambs fed the basal diet, but fish meal decreased (P < 0.01) the n-6 FA content only in Exp. 1. Soy meal and protected canola seed diets increased (P < 0.01) LT muscle n-6 FA content but did not affect long-chain n-3 FA content. Longissimus thoracis muscle long-chain n-3 FA were mainly deposited in structural phospholipid, rather than in storage triglyceride. In both Exp. 1 and Exp. 2, the ratio of n-6:n-3 FA in LT muscle was lowest (P < 0.01) in lambs fed fish meal supplement compared with all other treatments. Protected canola seed diet increased the ratio of n-6:n-3 FA (P < 0.01) and PUFA:saturated fatty acid (P < 0.03) content from those animals fed the basal, fish meal, and unprotected rapeseed diets in Exp. 2. This was due to an increase in muscle n-6 FA content, mainly linoleic acid, of both phospholipid (P < 0.001) and triglyceride (P < 0.01) fractions and not to an increase in muscle n-3 FA content. The results indicate that by feeding fish meal supplement, the essential n-3 FA can be increased while lowering the ratio of n-6:n-3 content in lamb meat to an extent that could affect nutritional value, attractiveness, and the economic value of meat.

Comparison of the color stability and lipid oxidative stability of fresh and vacuum packaged lamb muscle containing elevated omega-3 and omega-6 fatty acid levels from dietary manipulation.

A series of three experiments were conducted with second cross ([Merino×Border Leicester]×Poll Dorset) wether lambs to evaluate the effects of dietary treatments on manipulation of muscle long-chain (LC) omega-3 fatty acids (FA) on the color stability and oxidative stability of fresh and vacuum packaged lamb. At the end of 7-, 6- and 6-week experimental periods for experiments (Exp.) 1-3 respectively, lambs were slaughtered at a commercial abattoir. At 24 h post-mortem, muscle longissimus lumborum (LL) and longissimus thoracis (LT) were removed and evaluated for color and lipid oxidative stability under specified commercial storage and display condition. Of the dietary supplements used, fish meal and fish oil moderately (P<0.01) and markedly (P<0.001) increased muscle omega-3 FA content, while both protected canola seed (P<0.001) and protected sunflower meal protein significantly (P<0.02) increased muscle omega-6 FA content or ratio of omega-6/omega-3 of the longissimus muscle. In all experiments, the substantial increase (P<0.001) in muscle LC omega-3 and omega-6 FA had no consistent significant effect on color values (redness (a*), yellowness (b*) and lightness (L*)) for fresh and vacuum packaged lamb over a 6-day display period. Lipid oxidation, determined by the levels of thiobarbituric acid reactive substances (TBARS) indicated the enrichment of muscle polyunsaturated fatty acid (PUFA) levels in lambs did not produce significant differences resulting either from main treatment effects or for treatment×day×type interactions (where type was fresh and vacuum packaged). Present results demonstrated the color and lipid oxidative stability of lamb longissimus muscle during refrigerated display was not affected by enhanced levels of omega-3 and omega-6 FA due to dietary treatments.

Ontogenic and nutritional changes in circulating insulin-like growth factor (IGF)-I, IGF-II and IGF-binding proteins in growing ewe and ram lambs.

The ontogeny of the IGF endocrine system was investigated in 15 young lambs before and after weaning at 62 days of age. Before weaning, plasma IGF-I concentrations were higher in rams than ewes, and plasma concentrations of IGF-II and IGF-binding protein-3 (IGFBP-3) also tended to be higher in rams than in ewes. Feed intake of ewes and rams was restricted after weaning to remove sex differences in feed intake. Plasma concentrations of IGF-I and IGFBP-3 did not differ between rams and ewes at 100 days of age, but plasma IGF-II was higher in rams than in ewes at this time. Since circulating concentrations of GH were higher in rams than in ewes at 100 days of age, this implies that the restricted feed intake blocked the IGF-I and IGFBP-3 responses to GH. We conclude that sex differences in circulating IGF-I and IGFBP-3 concentrations in the growing lamb alter with age, and are not present when nutrition is restricted.

GH, GH-releasing factor and somatostatin in the growing lamb: sex differences and mechanisms for sex differences.

Factors contributing to sex differences in the somatotrophic axis were investigated in growing lambs. In the first experiment, circulating patterns of GH in venous blood, pituitary content of GH and GH mRNA, and median eminence (ME) contents of GH-releasing factor (GRF) and somatostatin (SRIF) were characterized in prepubertal ram and ewe lambs which were pair-fed to remove sex differences in feed intake. Mean and baseline plasma GH concentrations, GH pulse amplitude, and integrated plasma GH were greater in ram lambs than in ewe lambs, but GH interpulse interval did not differ between sexes. The pituitary GH content and ME contents of GRF and SRIF were greater in rams than in ewes, but steady-state levels of mRNA for GH in the pituitary gland did not differ between sexes. A second experiment investigated sex effects on the levels of SRIF in hypophysial portal blood, and found that these did not differ between sexes. We concluded that the presence of sexually dimorphic patterns of GH secretion in the growing lamb is independent of feed-intake differences between sexes. The lack of sex differences in circulating patterns of SRIF in portal plasma implies that there may be a difference in GRF secretion which may produce sexually dimorphic patterns of GH secretion in lamb.

Sexual dimorphism of circulating somatotropin, insulin-like growth factor I and II, insulin-like growth factor binding proteins, and insulin: relationships to growth rate and carcass characteristics in growing lambs.

The effects of sex and age on patterns of circulating somatotropin (ST) concentration and plasma IGF-I, IGF-II, insulin, and IGF binding protein-3 (IGFBP-3) were studied in ram, wether, and ewe lambs (n = 7 or 8) sampled at mean ages of 81 (I1) and 158 d (I2). Between 81 and 158 d of age, rams grew more rapidly than wethers (P < .01), and wethers grew more rapidly than ewes (P < .01). The sex differences in growth were reflected in empty body weight at slaughter: rams > wethers > ewes (P < .05). Mean plasma ST concentrations, ST pulse amplitude, and integrated plasma ST concentrations were greater (P < .05) in rams than in ewes at I1 and I2. Characteristics of the ST plasma profile in wethers were generally intermediate between those of rams and ewes. The interpulse interval was greater in ewes than in wethers at I2. The IGF-I and IGFBP-3 concentrations were greater in rams than in ewes at both sampling times. Plasma IGF-II was greater in ewes than in rams at I2. Mean plasma ST was approximately two thirds less at I2 than at I1 regardless of sex. Mean plasma ST and IGF-I at both ages were positively correlated with growth. Mean plasma ST at I2 was negatively correlated with fatness at slaughter. Sex and age significantly affected patterns of circulating ST and concentrations of IGF-I and IGFBP-3 in prepubertal growing lambs, under conditions for which growth rates and composition were also sexually dimorphic.

Maternal undernutrition during mid-pregnancy in sheep. Placental size and its relationship to calcium transfer during late pregnancy.

The aim of the present experiment was to determine the relationship between placental and fetal weight after placental growth had been retarded by maternal undernutrition. Placental weight and fetal weight were measured in single-lamb-bearing ewes which were well-fed throughout pregnancy, or severely undernourished between the 30th and 96th day of pregnancy. Placental transfer of calcium and whole-body metabolism of both glucose and Ca were measured during late pregnancy. The change in fleece-adjusted live weight between the 30th and 96th day of pregnancy was 99 (SE 9.8) and -146 (SE 9.6) g/d for the well-fed and undernourished ewes respectively. The condition score of well-fed ewes did not significantly change between the 96th (2.9 (SE 0.08)) and 140th (3.0(SE 0.13)) day of pregnancy, while it increased from 1.6 (SE 0.15) to 2.3 (SE 0.11) for the previously undernourished group. Undernutrition caused an increase (P less than 0.01) in placental weight measured on the 96th (21%) and 140th (30%) day of pregnancy. In contrast fetal growth was not significantly affected by maternal undernutrition. While the voluntary dry matter intakes (g/d) of previously undernourished ewes after the 97th day of pregnancy were higher than for their well-fed counterparts, there was no significant difference between whole-body glucose or Ca metabolism, or the placental transfer of Ca measured during late pregnancy. This experiment confirms earlier reports of an increase in placental weight as a result of maternal undernutrition during mid-pregnancy; but the factors causing and the functional significance of this response have not been identified. Contrary to earlier proposals, placental weight per se did not limit fetal growth during late pregnancy. It is hypothesized that a combination of factors originating from maternal, placental and fetal sources act together to regulate growth of the fetus.

Threonine metabolism in sheep. I. Threonine catabolism and gluconeogenesis in mature Blackface wethers given poor quality hill herbage.

In three experiments, mature Blackface wethers were given freeze-stored Agrostis festuca herbage by continuous feeder. In Expt 1, on separate occasions [U-14C]threonine, [U-14C]glucose and NaH14CO3 were infused over 12 h periods to obtain estimates of irreversible loss rate (ILR) of threonine, glucose and carbon dioxide in the plasma and of the exchange of C between these metabolites. In Expts 2 and 3, during periods when glucose and threonine metabolism were examined, glucose loss across the kidneys (23-29 g/d) was induced by infusion of phloridzin. Results from the four sheep used in Expts 1 and 3 are presented as three-pool models. They indicate that threonine ILR (7.8 g/d; 3.1 g C/d) was approximately three times the estimated rate of absorption of exogenous threonine (1 g C/d). Glucose ILR was approximately 76 g/d (mean +/- SE; 30.3 +/- 0.57 g C/d). Only 0.3% of the glucose-C (0.09 g/d) was derived directly from threonine-C (i.e. 3% of the threonine-C ILR). Bicarbonate ILR was 170 +/- 7.3 g C/d, and glucose contributed 11.1 +/- 3.52 g C/d to this, accounting for 51 +/- 4.4% of glucose-C ILR. Threonine contributed 0.20 +/- 0.026 g C/d to the bicarbonate-C ILR, accounting for only 6.4 +/- 0.87% of the threonine-C ILR. When, in Expts 2 and 3, phloridzin was infused, glucose ILR was increased by 28 +/- 1.5% and bicarbonate ILR was increased by 13 +/- 2.4%. Threonine ILR (3.1 g C/d) was not increased, but the metabolic distribution of threonine-C was altered. The transfer of threonine-C into glucose and CO2 was increased by 39 and 69% respectively to 0.125 and 0.45 g C/d, accounting for 4 and 13% of the threonine ILR respectively. Both technical and metabolic considerations which affect interpretation of these results in terms of rates of catabolism of threonine and of quantitative estimates of gluconeogenesis from threonine are discussed.

Threonine metabolism in sheep. 2. Threonine catabolism and gluconeogenesis in pregnant ewes.

The irreversible loss rate (ILR) of glucose, bicarbonate and threonine were determined in six twin- and triplet-bearing ewes on three occasions during the last 6 weeks of gestation. Three of the ewes (group S) were given conventional rations of hay plus concentrates so that their blood ketone levels did not rise over this period. The other three ewes (group H) were given a fixed intake of hay throughout; their blood ketone levels rose, but remained at subclinical levels. The results are presented in the form of three-pool open-compartment models for each period. There was an increase in the glucose ILR for both groups over the 6-week period, but the over-all increase (mean +/- SE) was much greater (P less than 0.001) in group S (35.1 +/- 2.43 g carbon/d) than in group H (11.3 +/- 1.28 g C/d). Similarly, increases in bicarbonate ILR were also higher in group S (161 +/- 11.2 g C/d) than in group H (63 +/- 21.7 g C/d). However, whereas with group S ewes, receiving supplementary feed, this increase was progressive throughout the 6 weeks, with the group H ewes the increase which occurred over the last 3 weeks of gestation (56 +/- 26.3 g C/d) was much greater than that which occurred over the preceding 3 weeks (7 +/- 4.7 g C/d). This pattern was also evident in the oxidation of glucose to CO2. In contrast to the previously mentioned findings, neither threonine ILR nor the amount of threonine converted to glucose or catabolized to CO2 changed significantly over the 6-week period. The results are discussed in the light of findings presented in the previous paper that the amount of threonine used in catabolic processes can alter if a glucose-only sink is created in wether sheep.