Feeding value of corn silage estimated with sheep and dairy cows is not altered by genetic incorporation of Bt1376 resistance to Ostrinia nubilalis.

A genetically modified Bt176 corn hybrid (Rh208Bt)--providing control of European corn borer damage--and the conventional isogenic hybrid (Rh208)--harvested as whole plant silage--were evaluated in three separate feeding trials to verify that the in vivo feeding value was substantially equivalent among modified and conventional hybrids. In the first trial, after a week of preexperiment, two sets of six Texel sheep, housed in digestibility crates, were fed silage sources of Rh208 and Rh208Bt hybrids, and silage of three additional control varieties of low, intermediate, and high feeding value (Rh289, Adonis, and Adonis bm3) for 1 wk. Feed offered to sheep was adjusted to maintenance requirements based on metabolic body weight. Agronomic and biochemical traits were similar among the Rh208 and Rh208Bt hybrids. Organic matter digestibility (67.1 and 67.6%), crude fiber digestibility (52.9 and 54.2%), and neutral detergent fiber digestibility (50.2 and 49.0%) were not significantly different among Rh208 and Rh208Bt hybrids. In the second trial, two sets of 24 Holstein cows were fed silage from Rh208 and Rh208Bt corn hybrids for 13 wk, 9 wk after calving, and including 2 wk of preexperiment. Fat-corrected milk yield (31.3 and 31.4 kg/d), protein content (31.7 and 31.6 g/kg) and fat content (36.7 and 37.0 g/kg) in milk of dairy cows were unaffected by hybrid source. Body weight gains of cattle were not different. However, intake was significantly higher in cows fed Rh208Bt silage. In the third trial, five midlactation multiparous Holstein cows were successively fed the silage from Rh208 and Rh208Bt corn hybrids 2 or 3 wk. Data were considered only for the last week of each period. There were no significant effects on protein fractions, fatty acid composition, or coagulation properties of milk between Rh208 and Rh208Bt fed cattle. Cattle and sheep can perform equally well with a conventional or a genetically modified Bt176 corn silage.

Effects of rumen energy supply timing on feed intake control in lactating dairy cows.

The aim of this trial was to demonstrate the mechanisms and dynamics of signals triggered by digestion in the short-term control of feed intake. Large nylon bags containing rolled wheat were placed into the rumens of four fistulated cows--either prior to feed distribution or during a feeding period--and left for 4 h. To account for bag volume and its effects, bags full of indigestible sawdust were used as a control. The four treatments were compared according to a Latin square design with three replications. Regardless of their contents, when bags were present during the experimental feeding period, intake decreased by 1.2 kg of dry matter through a filling effect. Wheat in the bags had no specific effect on intake during the experimental intake period. Conversely, on the day after the experiment, the presence of wheat during feeding caused a 4.2-kg dry matter decrease in intake compared with the saw-dust control. This trial indicated that the nutritional feedback signaling effect on intake control during meals is delayed, contrary to that of rumen fill. Moreover, the delayed effect on intake is only observed when nutrient cues are synchronous with meals, and, consequently, could be the result of experience.

Effects of level and type of energy source (volatile fatty acids or glucose) on milk yield, composition and coagulating properties in dairy cows.

Four fistulated Holstein cows were arranged in a 4 x 4 Latin square design to study the effects of level and type of energy source on milk yield and composition. Treatments consisted of a basal diet fed alone (low energy treatment) or with 3.3 Mcal of net energy for lactation from extra nutrients perfused either into the rumen (either propionic acid or a mixture of volatile fatty acids) or into the duodenum (glucose). Increasing the energy input without changing the volatile fatty acid profile improved milk yield and slightly increased milk protein and fat yields. Compared with the isoenergetic mixture of volatile fatty acids, both propionic acid and glucose infusions significantly decreased fat content (-4.5 g/kg) and yields (respectively, -111 and -160 g/d), but affected fatty acid proportion and yield differently (more elongation process and less C18 with glucose infusion). Protein yield was slightly increased by propionic acid infusion but not by glucose because of the counterbalanced effects on milk yield (-1.3 kg/d) and protein content (1.5 g/kg). The coagulating properties of milk were directly linked to variations in protein, casein and mineral contents. In conclusion, propionic acid or glucose scarcely affected milk protein content, but induced a similar decrease in milk fat content probably through different metabolic pathways.

Factors contributing to variation in the proportion of casein in cows' milk true protein: a review of recent INRA experiments.

The aim of this study was to identify and rank the various factors, in particular those involving feeding, that affect the proportion of caseins in milk true protein. Twenty-nine feeding trials involving 821 lactations were assessed, and lactoprotein genetic variants were known for 551 of these. The main factor affecting the casein: protein ratio was the genetic variant of beta-lactoglobulin: once corrected for other factors, the milk of BB type animals had a ratio nearly 30 g/kg total protein higher than AA animals. kappa-Casein variant B also had a positive effect (+12 g/kg in favour of BB relative to AA animals). Except in the last weeks of pregnancy and the first weeks of lactation, the casein: protein ratio varied little during lactation. It was significantly reduced when milk cell count exceeded 200,000 cells/ml, even in the absence of clinical mastitis. It also decreased slightly with parity. Among the various dietary factors studied (level and type of nitrogen and energy supplies, forage type and preservation method), none had any significant effect on the milk casein: protein ratio, except in drastic dietary situations. That ratio increased very slightly in parallel with the animals' milk yield and milk protein content. In practice, measuring the milk protein content in animals free from clinical mastitis remains a very precise predictor of casein content, accounting for 93% of its variation.

Effects of graded duodenal infusions of glucose on yield and composition of milk from dairy cows. 1. Diets based on corn silage.

Four fistulated Holstein cows were arranged in a 4 x 4 Latin square design to study the effects of graded amounts of glucose (0, 500, 750, and 1500 g/d) infused in the duodenum on milk yield and composition and plasma metabolites. Cows were fed a basal diet of 50% corn silage, 17% dehydrated alfalfa, and 33% concentrate. The treatments (feed plus infusions) were isoenergetic. Increased amounts of glucose did not affect milk yield or protein content. Fat yield and content decreased in a curvilinear manner; the lowest fat content was obtained with about 750 g of glucose. The decrease in milk fat resulted from a reduced yield of long-chain fatty acids (C16 and C18), probably caused by lower mobilization of fat. The glucose treatments significantly affected profiles of medium-chain fatty acids, which promoted the elongation process. The most important change in the plasma concentration of amino acids concerned decreased branched-chain amino acids. The lactose content was not greatly affected by infusions of glucose despite a significant linear increase in the concentration of milk glucose. In conclusion, an increase in the supply of glucose had no effect on milk yield and had a slight, positive effect on protein yield but induced a dramatic decrease in fat yield.

Effects of a duodenal glucose infusion on the relationship between plasma concentrations of glucose and insulin in dairy cows.

The effects of duodenal infusion of glucose on the relationship between plasma concentrations of glucose and insulin and on milk composition were investigated in a crossover design. Eight dairy cows were continually infused with water (control) or glucose (1.5 kg/d). Cows received diets consisting of dehydrated whole-plant maize in restricted amounts to equalize the energy supply between treatments. Basal (before meal) plasma concentrations of glucose and insulin were increased, but concentrations of nonesterified fatty acids (NEFA) were decreased, by glucose treatment. During the first 2 h after feed distribution, plasma insulin increased, and plasma glucose and NEFA decreased, in both control and treated cows. Afterward, plasma glucose increased in treated cows but further decreased in control cows. The difference reached 8 mg/100 ml without any change in plasma insulin. During the meal, concentrations of growth hormones in plasma were inhibited to a similar extent in both groups. In response to intravenous glucose or insulin challenges, changes in plasma glucose, NEFA, and insulin stimulated by glucose were also very similar in both groups. In conclusion, duodenal infusion of glucose increased basal plasma concentrations of glucose and insulin, increased postprandial plasma glucose, and decreased NEFA without inducing insulin resistance. Glucose treatment did not change milk yield but decreased milk fat yield, mainly through a decrease in the yield of C18 fatty acids that were derived from circulating fatty acids. In the absence of insulin resistance, the decrease in the yield of C18 fatty acids might be attributed to an inhibition of adipose lipolysis or an increase in adipose lipogenesis.

[Effect of level and nature of degradable nitrogen on rumen digestion with a maize based silage diet in lactating cows]. / Effet du niveau et de la nature de l'azote dégradable sur la digestion ruminale d'un régime à base d'ensilage de maïs chez la vache laitière.

Four diets differing in level and source of nitrogen were given in a 4 x 4 latin square design using four dairy cows canulated in the rumen and duodenum. A low N diet (T-, 17.4 g N/kg DM) with only 17 g of degradable N per kg of fermentable OM (FOM) (covering 73% of microbial requirements) was supplemented with up to 21.6 g/kg DM and 22 g/kg FOM (covering 95% of microbial requirements) of increasing levels of preformed amino acids. Three mixtures of urea and casein [100:0 (U); 50:50 (UC) and 0:100 (C)] were used in order to supply 30, 15 and 0% of the total N of the diet as urea-N. The four diets (66% maize silage) were isoenergetic and were offered 95% of ad libitum twice daily. OM and NDF digestibilities in the total digestive tract and in the rumen did not differ between T- and U diets. These digestibilities increased linearly with the proportion of casein-N in the diet but the response remained low. The levels and the source of degradable N did not affect the non ammonia nitrogen (NNA) flow into the duodenum, the microbial N flow and the efficiency of microbial synthesis. NNA flow exceeded N intake for all diets but the net recycling into the rumen was markedly higher for the LN diet (+110 vs +31 g/day). This is related to a lower excretion of urea-N in urine (22 vs 80 g/day). These results suggest that in cows fed at 95% of ad libitum, the shortage of degradable N hardly affects the digestion of the diet and that the supply of preformed amino acids is not an important limiting factor for bacterial growth when maïze silage based diets are fed. Therefore, the supplementation with such preformed carbon chains is not of practical interest.

Lactational and systemic responses of dairy cows to postruminal infusions of increasing amounts of methionine.

Five multiparous Holstein cows were used in a study with a 5 x 5 Latin square design to measure the effects of postruminal infusion of Met on lactational performance and plasma metabolites. The treatments were duodenal infusions of 1) 10 g/d of Lys (control), 2) 10 g/d of Lys plus 6 g/d of Met, 3) 10 g/d of Lys pus 12 g/d of Met, 4) 10 g/d of Lys plus 18 g/d of Met, and 5) 10 g/d of Lys plus 24 g/d of Met. The cows were fed a diet of 61% maize silage, 31% concentrate, and 5% dehydrated alfalfa. The DMI were similar among treatments. Milk yield, 4% FCM, and milk fat yield and content were not affected by infusions. In contrast, milk protein yield and content were increased linearly as Met infusion increased, which was true also for plasma Met and Cys concentrations. Using measurements of AA flow to the duodenum and assumed intestinal digestibilities of 0.8 for digesta and 1.0 for infused AA, estimated concentrations of Lys and Met in total AA absorbed in the small intestine were 7.3% for Lys and 1.52, 1.73, 1.94, 2.15, and 2.36% for Met for diets 1 through 5, respectively. The substantial linear increases in milk protein yield and content indicated that postruminal Met supply was not adequate over the entire range of Met infusions. In conclusion, the extent of Met limitation in this study could be defined only as that exceeding 2.4% of total AA absorbed in the small intestine.

Effect of graded levels of duodenal infusions of casein on mammary uptake in lactating cows. 1. Major nutrients.

Effects of duodenally infused casein on milk secretion were investigated by coupling measurements of mammary blood flow and arteriovenous differences. Four lactating cows were given a continuous duodenal infusion of 0, 177, 352, and 762 g of calcium caseinate/d according to a Latin square design. Diets, formulated to meet 90% of energy and protein recommendations, consisted of a ratio of corn silage:concentrate of 76:24 supplemented with dehydrated alfalfa. Yields of milk, protein, and fat increased linearly with casein infusions (by up to 2.6 kg/d, 126 g/d, and 133 g/d, respectively) as did milk protein content (by up to 2.2 g/kg). Mammary blood flow and insulin, prolactin, and growth hormone in plasma were not affected by added casein. Concentrations of arterial acetate, glucose, BHBA, and NEFA remained unchanged, but their uptake by the udder increased by 3, 18, 22, and 91%, respectively, between the control and high casein treatments. Plasma arterial concentrations and the mammary uptake of essential and nonessential AA increased linearly with casein infusions. Although arterial concentrations increased to a much greater degree for essential AA than for nonessential AA, the increased udder extraction was higher for nonessential AA. The main part of the added AA (90%) was extracted by the udder. However, a direct transfer of supplementary-extracted AA into milk did not occur, thus increasing AA availability for other metabolic pathways.

Effect of infused volatile fatty acids and caseinate on milk composition and coagulation in dairy cows.

Milk protein secretion is changed by increasing the proportion of energy, mainly as propionic acid, or the availability of AA. Whether associative effects exist between energy nature and protein amounts is unknown. Therefore, ruminal isoenergetic infusions of low or high propionate mixtures were combined factorially with duodenal infusion of sodium caseinate or control. Four ruminally and duodenally fistulated Holstein cows were used. The diet was limited and consisted of 70% forage and 30% concentrate. Caseinate infusion increased milk yield and protein and casein contents and decreased milk fat content; curd yields and coagulation properties of milk were improved. The infusion of propionic acid caused a large increase in rumen propionate. Milk yield tended to decrease, and milk fat decreased, but protein, casein, and curd yields were unchanged; milk-coagulating properties were improved. No interaction existed between energy and protein amounts. Alteration of VFA had little effect on milk composition, but increasing the protein supply to the duodenum increased milk protein.

[Degradability in small bags of nitrogenous components of feed concentrates: standardization of the method and intra- and interlaboratory variability]. / Dégradabilité en sachets des matières azotées des aliments concentrés: standardisation de la méthode et variabilités intra- et interlaboratoires.

In sacco degradability of feed protein (DT) was measured in 13 feedstuffs by 4 laboratories according the French standardized procedure. Adjustment for variations in microbial activity by reference to DM disappearance of a standard sample of lucerne decreased within and between lab variability. When the standardized procedure was carefully followed, results were in rather good agreement between labs.

[Influence of dietary factors on microbial protein synthesis in the rumen]. / Influence des facteurs alimentaires sur la protéosynthèse microbienne dans le rumen.

The effect of dietary factors (usually controlled in practice) on microbial protein synthesis is reviewed using in vivo experiments. Attention is drawn on the necessity to clearly distinguish variations in microbial growth efficiency from those of intestinal flow of microbial protein and to consider simultaneously variations in feed protein degradation. In practice, the relationship between microbial protein synthesis and energy intake depends mainly on diet composition and the nature of the forage. Microbial protein flow to the intestine, relative to energy intake, is lower with high concentrate diets (when given in restricted amounts), with silages and with antibiotic supplements. This flow is increased by some forage processing (such as dehydration and alkali treatments), by natural or induced defaunation, and occasionally by increased feeding frequency (when intake is restricted) and buffer and vitamin supplements. However, with some factors such as feeding frequency and antibiotics supplementation, these variations are partly counterbalanced by reverse effects on feed protein degradation.