Graduate Student Literature Review: System, plant, and animal factors controlling dietary pasture inclusion and their impact on ration formulation for dairy cows.

Diet formulation in a pasture-based dairy system is a challenge as the quality and quantity of available pasture, which generally constitutes the base diet, is constantly changing. The objective of this paper is to cover a more in-depth review of the nutritional characteristics of pasture-based diets, identifying potential system, plant, and animal factors that condition pasture dietary inclusion in dairy cows. In practice, there is a wide diversity of pasture-based systems with predominant to minimal use of pasture requiring a more specific classification that potentially considers the amount and time of access to pasture, access to housing, length of grazing season, seasonality of calving, and level and method of supplementation. There are important differences in the nutritional quality between pasture species and even cultivars. However, under management practices that promote maintenance of pasture in a vegetative state as well as controlling the availability of pasture, it is possible to achieve high dry matter intakes (∼2.9%-3.4% of live weight) of pasture with moderate to high diet energy density, protein supply, and digestibility. The amount of pasture to include in the diet will depend on several factors, such as the type of production system, the cost of supplementary feeds, and the farmer's objectives, but inclusions of ∼40% to 50% of the diet seem to potentially reduce costs while apparently not limiting voluntary feed intake. Considering that there seems to be a continuum of intermediate management systems, a better understanding of the factors inherent to the feed ingredients used, as well as the use of nutrients by cows, and potential interactions between animal × system should be addressed in greater depth. This requires a meta-analysis approach, but given the diversity of the pasture-based system in practice, the existing information is highly fragmented. A clear definition of "subsystems" is necessary to direct the future research and development of mechanistic models.

Assessing amino acid uptake and metabolism in mammary glands of lactating dairy cows intravenously infused with methionine, lysine, and histidine or with leucine and isoleucine.

The objective of this study was to evaluate the effect of jugular infusions of 2 groups of AA on essential AA (EAA) transport and metabolism by mammary glands. Four Holstein cows in second lactation (66 ± 10 d in milk) were used in 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments. Treatments were jugular infusions of saline; Met, Lys, and His (MKH); Ile and Leu (IL); or MKH plus IL (MKH+IL). Each period consisted of 8 d of no infusion followed by 8 d of jugular vein infusion of the treatment solutions. Amino acids were infused at rates of 21 g of Met, 38 g of Lys, 20 g of His, 50 g of Leu, and 22 g of Ile per day. Cows were fed a basal diet consisting of 15.2% crude protein with adequate rumen degradable protein but 15% deficient in MP based on estimates by Cornell Net Carbohydrate and Protein System (v6.5). On the last day of each period, 13C-AA derived from algae was infused into the jugular vein over 6 h, and blood and milk samples were collected before, during, and after infusion. Plasma and milk samples were analyzed for AA isotopic enrichment, and a mammary compartmental model was fitted to the data to derive bidirectional transport and metabolism rates for individual EAA. Influx of Leu increased with IL, whereas influx of other EAA was not different among treatments. Cellular efflux of Met and Lys to venous plasma represented 12 to 34% of influx, whereas cellular efflux of Phe and BCAA represented 29 to 59% of influx. Increased efflux/influx ratios of Ile and Leu with IL but not Met and Lys with MKH demonstrated that increased Ile and Leu influx was mostly returned to plasma resulting in no change in net uptake or efficiency. The isotope results showed that mammary net uptake of Lys and Ile increased during MKH infusion. Net uptake of Met increased with MKH but only in the absence of IL. Catabolism of Lys and Met only increased with MKH alone, resulting in decreased efficiency for milk protein, which demonstrated that Ile and Leu infusion can spare Lys and Met for milk protein synthesis. Total AA uptake to milk output was not different from 1, implying the catabolized Met and Lys contributed nitrogen to nonessential AA. Overall, EAA uptake and metabolism in mammary glands of dairy cows varied across individual EAA and responded differently to respective AA supplements. In addition, uptake, retention, and end use of AA by mammary tissue is variable and dependent on the mix of AA provided. This variability, depending on the mix of AA absorbed, will change the efficiency of utilization of individual AA at the mammary gland level and consequently the whole-body level. Thus, it is inaccurate to use a fixed, constant efficiency within and across AA to represent tissue activity.

Net mineral requirements of dairy goats during pregnancy.

Mineral requirements of pregnant dairy goats are still not well defined; therefore, we investigated the net Ca, P, Mg, Na and K requirements for pregnancy and for maintenance during pregnancy in two separate experiments. Experiment 1 was performed to estimate the net Ca, P, Mg, Na and K requirements in goats carrying single or twin fetuses from 50 to 140 days of pregnancy (DOP). The net mineral requirements for pregnancy were determined by measuring mineral deposition in gravid uterus and mammary gland after comparative slaughter. In total, 57 dairy goats of two breeds (Oberhasli or Saanen), in their third or fourth parturition, were randomly assigned to groups based on litter size (single or twin) and day of slaughter (50, 80, 110 and 140 DOP) in a fully factorial design. Net mineral accretion for pregnancy did not differ by goat breed. The total daily Ca, P, Mg, Na and K requirements for pregnancy were greatest in goats carrying twins (P<0.05), and the requirements increased as pregnancy progressed. Experiment 2 was performed to estimate net Ca, P, Mg, Na and K requirements for dairy goat maintenance during pregnancy. In total, 58 dairy goats (Oberhasli and Saanen) carrying twin fetuses were assigned to groups based on slaughter day (80, 110 and 140 DOP) and feed restriction (ad libitum, 20% and 40% feed restriction) in a randomized block design. The net Ca, P and Mg requirements for maintenance did not vary by breed or over the course of pregnancy. The daily net requirements of Ca, P and Mg for maintenance were 60.4, 31.1 and 2.42 mg/kg live BW (LBW), respectively. The daily net Na requirement for maintenance was greater in Saanen goats (11.8 mg/kg LBW) than in Oberhasli goats (8.96 mg/kg LBW; P<0.05). Daily net K requirements increased as pregnancy progressed from 8.73 to 15.4 mg/kg LBW (P<0.01). The findings of this study will guide design of diets with adequate mineral content for pregnant goats throughout their pregnancy.

Effects of different supplemental soya bean oil levels on the performance of prepubertal Saanen goats: Oestrogen and progesterone release.

The aim of this study was to investigate the effects of different levels of soya bean oil in the total diet on the growth rate, metabolic changes, and oestrogen and progesterone release in Saanen goats. After dietary adaptation, 21 prepubertal goats (weight of 29.12 ± 0.91 kg, 230 days old) were randomly distributed among three diets of D2: inclusion of 2% soya bean oil in the total diet; D3: basal diet - inclusion of 3% soya bean oil in the total diet; and D4: inclusion of 4% soya bean oil in the total diet. The basal diet (D3) was formulated to promote a daily gain of 0.140 kg. The goats were weighed, and their blood samples were collected weekly. Glucose, cholesterol, triglycerides, total protein, urea, non-esterified fatty acids, beta-hydroxybutyrate, oestrogen and progesterone in the plasma were measured. Prepubertal goats that were fed D4 exhibited a significantly lower dry matter intake, urea and cholesterol levels compared with the goats that were fed D2 and D3. Indeed, goats that were fed D4 displayed a significantly lower final weight than goats that were fed D2 and D3. In contrast, the inclusion of soya bean oil in the diet increased the progesterone and oestrogen concentrations, and goats that were fed D4 released a significantly higher concentration of progesterone than those that were fed D2 and D3. Furthermore, the percentage of goats with a progesterone level greater than 1 ng/ml (functional Corpus luteum) was significantly higher among the goats that were fed D3 and D4 than among those that were fed D2. In this study, although the inclusion of 4% soya bean oil in the diet decreased dry matter intake and growth rate, it increased progesterone concentration and the percentage of goats with a functional Corpus luteum, suggesting that the inclusion of soya bean oil accelerated puberty in prepubertal goats.

Mineral requirements for growth and maintenance of F1 Boer × Saanen male kids.

The objective of this study was to determine the net requirements of minerals for the growth and maintenance of intact male F1 Boer × Saanen goat kids in the initial phase of growth. The following 2 experiments were performed: Exp. 1 was performed to determine the net growth requirements for Ca, P, Mg, Na, and K by F1 Boer × Saanen goat kids from 5 to 25 kg of BW and Exp. 2 was performed to determine the maintenance requirements of F1 Boer × Saanen goats from 15 to 25 kg BW. In Exp. 1, 32 intact male goat kids were distributed in a completely randomized design and mineral body composition was fit to an allometric equation in the form of a nonlinear model. To determine the mineral requirements for maintenance in Exp. 2, 21 intact male goat kids were distributed in a randomized block design, where the goat kids were subjected to 3 levels of feed restriction (0, 30, and 60% feed restriction). At the onset of Exp. 2, 7 goat kids were harvested and used to estimate the initial body composition (15 kg BW). Initial body composition was used to calculate the retention of minerals. The maintenance requirements were estimated by regressions obtained from the retention of minerals in the empty body and the intake of the mineral. The concentration of Ca, P, Na, and K in the empty BW decreased by 11, 13, 26, and 23% with the increase in BW from 5 to 25 kg (P < 0.01). As a consequence, our results showed that net requirements of Ca, P, Mg, Na, and K for weight gain decreased by 27.5, 27.8, 4.25, 43.2, and 39.7%, respectively, with the increase in BW from 5 to 25 kg (P < 0.01). The net requirements (g/kg of ADG) decreased from 9.7 to 7.0 for Ca, 6.5 to 4.7 for P, 0.38 to 0.36 for Mg, 0.88 to 0.50 for Na, and 1.9 to 1.2 for K when BW increased from 5 to 25 kg. The daily net requirements for maintenance per kilogram of BW were 38 mg of Ca, 42 mg of P, 1.6 mg of Mg, 5.0 mg of Na, and 19 mg of K. These results for the nutritional requirements of minerals may help to formulate more balanced diets for F1 Boer × Saanen goat kids in the initial growth phase.