Nitrogen utilization in grazing cows is often low due to high concentrations of rapidly soluble and degradable protein in the pasture-based diet. Broadly, opportunities to improve N utilization lie in either reducing the amount of N consumed by the animal, or incorporating more N into milk protein. The goal of this study was to compare the relative importance of dietary N intake and productive N output for improving N utilization in grazing cows fed either starch-, fiber-, or sugar-based supplements. Also, the Cornell Net Carbohydrate and Protein System (CNCPS; Cornell University, Ithaca, NY) was evaluated as a tool to assess cow performance and improve N utilization in pasture-based systems. Eighty-five cows were randomly assigned to 1 of 5 treatments at parturition (17 cows per treatment). Treatments consisted of a pasture-only control and pasture with a starch- (St and StN), fiber- (FbN), or a sugar-based supplement. The StN and FbN treatments contained additional dietary N. Diets were formulated using the CNCPS to supply similar levels of dietary metabolizable energy, but differing levels of dietary N and metabolizable protein. Nitrogen utilization ranged from 22 to 26% across the 5 groups. Cows fed the St diet had the lowest levels of milk urea N, blood urea N, and urinary N excretion and had the highest productive N output (149 g/d). Cows fed the FbN treatment had similar productive N output (137 g/d) and consumed approximately 100g/d more dietary N than the St treatment, resulting in greater urinary N excretion. Although milk protein yield was moderately greater in the St treatment, quantitatively the difference in N intake (100g/d) had the greatest effect on N utilization and suggests that controlling dietary N intake should be the first priority when attempting to improve N utilization in grazing cows. No effect was observed of supplementing pasture-fed cattle with sugar on production or N utilization under the conditions of this experiment. Predictions of metabolizable energy and protein availability for milk yield from the CNCPS were similar to actual milk yield for all treatments. Model-predicted N utilization and excretion reflected the trends observed in the measured data and suggests that the CNCPS can be a useful tool for formulating and evaluating diets to improve N utilization in pasture-based systems.

Fifty-three nonlactating, pregnant Holstein-Friesian and Holstein-Friesian × Jersey cross dairy cows were grouped into 4 cohorts (n=15, 12, 13, and 13) and offered 1 of 3 allowances of fresh, cut pasture indoors for 38 ± 2 d (mean ± SD). Cows were released onto a bare paddock after their meal until the following morning. Animals were blocked by age (6 ± 2 yr), day of gestation (208 ± 17 d), and body weight (BW; 526 ± 55 kg). The 3 pasture allowances [low: 7.5 kg of dry matter (DM), medium: 10.1 kg of DM, or high: 12.4 kg of DM/cow per day] were offered in individual stalls to determine the estimated DM and metabolizable energy (ME) intake required for zero energy balance. Individual cow DM intake was determined daily and body condition score was assessed once per week. Cow BW was recorded once per week in cohorts 1 and 2, and 3 times per week in cohorts 3 and 4. Low, medium, and high allowance treatments consumed 7.5, 9.4, and 10.6 kg of DM/cow per day [standard error of the difference (SED)=0.26 kg of DM], and BW gain, including the conceptus, was 0.2, 0.6, and 0.9 kg/cow per day (SED=0.12 kg), respectively. The ME content of the pasture was estimated from in vitro true digestibility and by near infrared spectroscopy. Total ME requirements for maintenance, pregnancy, and limited activity were 1.07 MJ of ME/kg of measured metabolic BW per day. This is more than 45% greater than current recommendations. Differences may be due to an underestimation of ME requirements for maintenance or pregnancy, an overestimation of diet metabolizability, or a combination of these. Further research is necessary to determine the reasons for the greater ME requirements measured in the present study, but the results are important for on-farm decisions regarding feed allocation for nonlactating, pregnant dairy cows.

Four cohorts of nonlactating, pregnant dairy cows (n=50, 47, 45, and 42) were individually fed indoors to determine the amount of feed required for body weight (BW) gain from autumn pasture and commonly used supplementary feeds. These results were used to estimate the apparent efficiency with which metabolizable energy (ME) is used for BW gain (app_kg). Control cows were offered autumn pasture to estimated maintenance requirements (~0.55 MJ of ME/kg of BW(0.75)), with an additional 20 MJ of ME/d allocated for pregnancy and activity. All other cows received the same allowance of autumn pasture and an additional allowance (2.5 or 5.0 kg of dry matter/d) of autumn pasture (Past), spring pasture silage (Psil), maize silage (Msil), cracked maize grain (Mgr), or palm kernel expeller (PKE), resulting in a total of 11 treatments. Individual cow dry matter intake was determined daily; BW was recorded once per week for cohorts 1 and 2, and 3 times per week for cohorts 3 and 4. The ME contents of feeds were estimated from feed quality assays. Regression analyses were used on each feed to determine the ME requirement for 1 kg of BW gain. The app_kg of Past and Msil was 0.34 and 0.47, respectively; these estimates are in line with published literature. The app_kg of Psil (0.50) was consistent with the published kg for spring pasture, from which the silage was made. Palm kernel expeller had the greatest app_kg (0.61). The reasons for this cannot be deduced from the current study but may reflect the relatively high fat content of the feed and the high kg of fat. The app_kg for Mgr was low (0.38) in comparison with the other supplementary feeds and, in particular, relative to its feed ME and published kg estimates. Although the reason for the low app_kg cannot be deduced from the current data, the most plausible reason is the preferential use of propionate-derived glucose for conceptus metabolism rather than BW gain, a factor not accounted for in previous experimental models that did not use late-gestation cows. In summary, the app_kg for autumn pasture was low but consistent with historical growth rate trials in other ruminant species. In comparison, Msil, Psil, and PKE were used with a greater apparent efficiency (app_kg=0.47 to 0.61), but Mgr resulted in a relatively low rate of gain per MJ of ME (app_kg=0.38). These differences have implications for accurate feed budgeting on farm.

The objective of this experiment was to determine if transcript abundance of genes involved in metabolic pathways in adipose and liver tissue could provide some explanation for the low efficiency with which ME in autumn pasture is used for BW gain. Nonlactating, pregnant (208 ± 19 d of gestation or approximately 75 d precalving) dairy cows (n = 90) were randomly allocated to either a control diet (i.e., offered fresh autumn pasture to maintenance requirements: 0.55 MJ ME/kg of measured metabolic BW [BW0.75] per day) or, in addition to the control diet, 1 of 2 supplement amounts (2.5 and 5.0 kg DM/d) of autumn pasture or 1 of 4 supplementary feeds (i.e., a control and 2 levels of feeding for each of 5 feeds: 11 groups of cows). Along with autumn pasture, evaluated feeds included spring pasture silage, maize silage, maize grain, and palm kernel expeller. Adipose and liver tissues were biopsied in wk 4 of the experiment and transcript abundance of genes involved in metabolic pathways associated with energy metabolism, lipolysis, and lipogenesis was determined. Additional feed, irrespective of type, increased BW gain (P < 0.01) and this effect was reflected in the expression of genes in adipose and liver tissue. However, autumn pasture had lower energy-use efficiency than the other feeds. Genes involved in both lipogenesis (ACACA, THRSP, GPAM, GPD1, and LPL) and lipolysis (PNPLA2) were upregulated (P < 0.05) in adipose tissue in response to increased ME intake/kilogram BW0.75. Hepatic expression of APOA1 decreased and that of APOB increased (P < 0.05) in cows offered maize grain and maize silage (i.e., starch-containing feeds). In comparison, pasture-fed cows demonstrated a degree of uncoupling of the somatotropic axis, with lower hepatic transcript abundance of both GHR1A and IGF-1 compared with cows offered any of the other 4 feeds. Changes to gene transcription indicate a possible molecular mechanism for the poor BW gain evident in ruminants consuming autumn pasture.