P. C. Garnsworthy

Fertility in the high-producing dairy cow

Abstract Genetic correlations between milk yield and reproductive measures in dairy cows are unfavourable. This suggests that successful selection for higher yields may have led to a decline in fertility. There is also evidence that an imbalance of nutrients, in either high genetic merit cows or those fed diets not matched to their performance, leads to poorer reproductive performance. Physiological reasons for the antagonism have not been elucidated. In this paper we examine the complexity of genetic, nutritional, physiological and management factors of the yield versus fertility antagonism. To maintain or recover high fertility in modern dairy cows calls for a two-pronged approach involving both inclusion of fertility in broader breeding goals and adjustment to management practices.

Seasonal variation in milk conjugated linoleic acid and Δ9-desaturase activity in dairy cows

The objective of this work was to study changes in the fatty acid profile of cows’ milk throughout the year with particular emphasis on cis-9, trans-11 conjugated linoleic acid (CLA), a proven anticarcinogen, found predominantly in milk and meat from ruminants. During the winter months, a total mixed ration of grass and maize silages, brewers grains, cereals, soya and dairy concentrates was fed. Through the summer months, fresh grass was fed, with increasing levels of buffer feeding given as the summer progressed. The CLA content of milk in May, June and July was significantly higher (P<0.05) than all other months, averaging 1.50 g CLA/100 g FAME compared with a mean of 0.77 g/100 g for the other months. Δ9-Desaturase activity was also greater in the summer. Milk fat produced during the summer contained significantly (P<0.05) greater amounts of short-chain fatty acids at the expense of medium-chain fatty acids indicating that fresh grass may alter the pattern of fatty acids produced de novo in the mammary gland. Results suggest that fresh grass promotes the synthesis of CLA in the dairy cow through an increase in Δ9-desaturase activity in the mammary gland and possibly other unknown factors.

Impact of Dietary Fatty Acids on Oocyte Quality and Development in Lactating Dairy Cows

Abstract The purpose of this study was to examine the effects of level of rumen inert fatty acids on developmental competence of oocytes in lactating dairy cows. Estrous cycles were synchronized in 22 cows on a silage-based diet supplemented with either low (200 g/day) or high (800 g/day) fat. A total of 1051 oocytes were collected by ultrasound-guided ovum pickup (OPU) in seven sessions/cow at 3–4 day intervals. Oocytes were matured, fertilized, and cultured to the blastocyst stage in vitro. Embryo quality was assessed by differential staining of Day 8 blastocysts. The high-fat diet reduced numbers of small and medium follicles. There was no effect on the quality of oocytes (grades 1–4) or cleavage rate. However, high fat significantly improved blastocyst production from matured (P < 0.005) and cleaved (P < 0.05) oocytes. Blastocysts from the high-fat group had significantly more total, inner cell mass and trophectoderm cells than the low-fat group (P < 0.05). Regression analysis showed negative effects of milk yield (P < 0.001), dry matter intake (P < 0.001), metabolizable energy intake (P < 0.005), and starch intake (P < 0.001) on blastocyst production in the low-fat group but not in the high-fat group. Within the low-fat group, blastocyst production was negatively related to growth hormone (P < 0.05) and positively related to leptin (P < 0.05). The low-fat group had higher nonesterified fatty acids than the high-fat group (P < 0.05). In conclusion, higher milk yields were associated with reduced developmental potential of oocytes in cows given a low-fat diet. Provision of a high-fat diet buffered oocytes against these effects, resulting in significantly improved developmental potential.

Effects of synchronizing the rate of dietary energy and nitrogen release in diets with a similar carbohydrate composition on rumen fermentation and microbial protein synthesis in sheep

The effects of synchronizing dietary energy and nitrogen supply in diets with a similar carbohydrate composition on microbial protein synthesis and rumen fermentation were examined in sheep. Two diets were formulated to be either synchronous (diet S) or asynchronous (diet A) for the hourly release of nitrogen (N) and energy to the rumen. Diet S contained (g/kg) 425 g wheat straw, 400 g winter barley, 150 grapeseed meal and 25 g minerals/vitamins and diet A contained 505 g wheat straw, 458.5 g winter barley, 11·5 g urea and 25 g minerals/vitamins. Both diets were fed at the rate of 1 kg/day in four equal portions, to four cannulated sheep, in two periods in a change-over design. Rumen ammonia concentrations followed the predicted hourly trend in N degradation with a peak 1 h after feeding of 10 mM for diet S and 16 mM for diet A before falling within 3 h of feeding to 4 ITIM in animals fed either diet. Rumen volatile fatty acid (VFA) concentrations followed the cyclical trend predicted by stoichiometric equations, whilst rumen VFA ratios were more stable than predicted in animals fed either diet. The observed content of rumen degradable protein and organic matter truly degraded in the rumen was similar for both diets. The increase in total CHO digested in the rumen observed with diet A (427 g/kg DM) compared with diet S (364 g/kg DM) can be attributed to the greater content of starch in the asynchronous diet, which had a high degradability. The efficiency of microbial protein synthesis (g N/kg OM truly degraded in the rumen) was 11–20% greater in animals fed the synchronous diet (S) than the asynchronous diet (A). It is concluded that microbial N production was more efficient when dietary energy and N supply were synchronized.

Effect of dietary-induced changes in plasma insulin concentrations during the early post partum period on pregnancy rate in dairy cows

Dietary stimulation of insulin in post partum dairy cows has been found to enhance ovarian follicle development but to impair oocyte developmental competence. The objective of this study was to test the hypothesis that pregnancy rate would be improved by feeding a diet to stimulate higher insulin (H) until cows resumed ovarian cyclic activity after parturition, and then feeding a diet to lower insulin (L) during the mating period. Each diet was fed to 30 post partum dairy cows until their first rise in milk progesterone, when 15 cows in each group were transferred to the other diet (treatments HL and LH) and 15 cows in each group remained on their original diet (treatments HH and LL) until 120 days post partum. Treatments did not affect dry matter intake, milk yield and metabolisable energy balance. Plasma insulin concentration was elevated in cows fed on H compared with cows fed on L. Treatment did not affect days to first progesterone rise, first oestrus or first insemination. At 120 days post partum, 27% of cows on each of treatments HH, LL and LH were pregnant, but 60% of cows on treatment HL were pregnant (P=0.021). These findings support the concept that physiological relationships between insulin and the reproductive system vary according to stage of the reproductive cycle, and suggest that pregnancy rate can be enhanced by a two-diet strategy tailored to optimise responses before and after the first post partum ovulation.

Dietary omega-3 and -6 polyunsaturated fatty acids affect the composition and development of sheep granulosa cells, oocytes and embryos

The evidence that omega-3 (n-3) and -6 (n-6) polyunsaturated fatty acids (PUFAs) have differential effects on ovarian function, oocytes and embryo quality is inconsistent. We report on the effects of n-3 versus n-6 PUFA-enriched diets fed to 36 ewes over a 6-week period, prior to ovarian stimulation and follicular aspiration, on ovarian steroidogenic parameters and embryo quality. Follicle number and size were unaltered by diet, but follicular-fluid progesterone concentrations were greater in n-3 PUFA-fed ewes than in n-6 PUFA-fed ewes. The percentage of saturated FAs (mostly stearic acid) was greater in oocytes than in either granulosa cells or plasma, indicating selective uptake and/or de novo synthesis of saturated FAs at the expense of PUFAs by oocytes. High-density lipoproteins (HDLs) fractionated from sera of these ewes increased granulosa cell proliferation and steroidogenesis relative to the FA-free BSA control during culture, but there was no differential effect of n-3 and n-6 PUFAs on either oestradiol or progesterone production. HDL was ineffective in delivering FAs to embryos during culture, although n-6 PUFA HDL reduced embryo development. All blastocysts, irrespective of the treatment, contained high levels of unsaturated FAs, in particular linoleic acid. Transcripts for HDL and low-density lipoprotein (LDL) receptors (SCARB1 and LDLR) and stearoyl-CoA desaturase (SCD) are reported in sheep embryos. HDL reduced the expression of transcripts for LDLR and SCD relative to the BSA control. The data support a differential effect of n-3 and n-6 PUFAs on ovarian steroidogenesis and pre-implantation development, the latter in the absence of a net uptake of FAs.

Integration of physiological mechanisms that influence fertility in dairy cows

Fertility in dairy cows has been declining for the past three decades. Genetic selection for increased milk production has been associated with changes in key metabolic hormones (growth hormone, insulin, IGF and leptin) that regulate metabolism by homoeostasis and homeorhesis. These metabolic hormones, particularly insulin, provide signals to the reproductive system so that regulation of ovarian function is coordinated with changes in metabolic status. Studies have shown, for example, that increasing circulating insulin concentrations during the early post partum period can advance the resumption of oestrous cycles by enhancing follicular growth. However, high concentrations of insulin can be detrimental to the developmental competence of oocytes, which is also influenced by the supply of fatty acids at the systemic level and at the ovarian level. Insulin status is also associated with the incidence and characteristics of abnormal ovarian cycles. These changes can occur without significant variation in circulating gonadotrophin concentrations. This suggests that additional factors, such as peripheral metabolites, metabolic hormones and locally produced growth factors, may have a modulating role. Recent evidence has demonstrated that ovarian responses to metabolic signals and nutrient profile vary according to the stage of the reproductive cycle. Improved understanding of this multifactorial process enables nutrition to be matched to genotype and milk production, with a positive impact on pregnancy rate.

On-farm methane measurements during milking correlate with total methane production by individual dairy cows

The objective of this study was to investigate whether measurement of methane emissions by individual dairy cows during milking could provide a useful technique for monitoring on-farm methane emissions. To quantify methane emissions from individual cows on farm, we developed a novel technique based on sampling air released by eructation during milking. Eructation frequency and methane released per eructation were used to estimate methane emission rate. For 82 cows, methane emission rate during milking increased with daily milk yield (r = 0.71), but varied between individuals with the same milk yield and fed the same diet. For 12 cows, methane emission rate recorded during milking on farm showed a linear relationship (R² = 0.79) with daily methane output by the same cows when housed subsequently in respiration chambers. For 42 cows, the methane emission rate during milking was greater on a feeding regimen designed to produce high methane emissions, and the increase compared with a control regimen was similar to that observed for cows in respiration chambers. It was concluded that, with further validation, on-farm monitoring of methane emission rate during milking could provide a low-cost reliable method to estimate daily methane output by individual dairy cows, which could be used to study variation in methane, to identify cows with low emissions, and to test outcomes of mitigation strategies.

The effect of supplementing grass silage with barley on digestibility, in sacco degradability, rumen fermentation and methane production in sheep at two levels of intake

Two experiments are reported which examined the digestibility, in sacco degradability, rumen fermentation and methane production of grass silage unsupplemented or supplemented with rolled barley at three ratios and two planes of nutrition. Grass silage was given with rolled barley to four mature wether sheep using forage:concentrate (F:C) ratios of 1.00, 0.75, 0.50, and 0.25 (dry matter (DM) basis) at near to maintenance (experiment 1) and 1.5 × maintenance (experiment 2) in two consecutive latin square designs. All diets within plane of nutrition were designed to be isoenergetic and all animals received a mineral/vitamin supplement. The diets were fed for 28 d, with rumen fluid sampling on day 12. Apparent digestibility and rumen degradability (in sacco) of diet organic matter (OM) were measured and methane produced was measured in open-circuit respiration chambers for four consecutive 24 h periods. Apparent digestibility of organic matter (OMD) increased linearly (P < 0.001) in both experiments with decreasing F:C ratio (0.74, 0.79, 0.83, 0.88 and 0.72, 0.75, 0.78, 0.82 for experiments 1 and 2 respectively). Decreasing F:C ratio depressed the effective degradability of silage OM from polyester fibre bags in both experiments (quadratic effect P < 0.05) and also for barley in experiment 1 (quadratic effect P < 0.05). Rumen pH was significantly decreased with decreasing F:C ratio in both experiments as was the total mean VFA concentration. The molar proportions of acetate and propionate tended to decrease and that of butyrate increased with decreasing F:C ratio. Over the range of barley inclusion in this study there were no associative effects on methane production (1 d−1) in experiment 1 (37.2, 41.5, 41.9, 44.0 1 d−1 for F:C ratio 1.00, 0.76, 0.51 and 0.26 respectively), but the opposite was true at the higher level of intake (methane 1 d−1, 54.1, 60.1, 58.0, 50.1 for F:C ratios 1.00, 0.78, 0.54, 0.28 respectively). Rumen stoichiometry could not be used to explain the change in methane production. Increased level of intake reduced methane production (1 kg−1 organic matter apparently digested) regardless of F:C ratio. Expressing methane relative to the efficiency of digestion, or the efficiency of animal production, enables the potential for reduction of methane generation by starch supplementation and/or level of intake to be realised.

Nutrition, Metabolism, and Fertility in Dairy Cows: 1. Dietary Energy Source and Ovarian Function

In previous studies, high plasma insulin was associated with earlier resumption of postpartum estrous cycles in dairy cows. The objective of this experiment was to quantify hormonal and ovarian responses to dietary starch and fat contents. Thirty cows were fed on a standard diet from calving until 40 d in milk (DIM) and then 6 cows were allocated to each of 5 isoenergetic diets containing 231, 183, 159, 135, and 87 g of starch and 39, 42, 43, 45, and 48 g of fat/kg of dry matter (DM) for diets 1 to 5, respectively, until 70 DIM. Estrus was synchronized at 60 DIM. Between 60 and 70 DIM, energy intake, milk yield, and energy balance were similar among diet groups. Plasma insulin-to-glucagon ratio increased with increasing dietary starch and decreasing dietary fat concentrations, reaching a break point at 159 g of starch, 43 g of fat/kg of DM (diets 1 to 5: mean 3.86, 3.78, 3.59, 2.98, 2.06 +/- standard error 0.22). Growth hormone, insulin-like growth factor-I, and leptin did not vary among diets. The greatest dietary starch concentration was associated with elevated plasma urea-N (diets 1 to 5: mean 3.69, 3.01, 2.94, 2.95, 2.75, +/- standard error 0.13 mmol/L, respectively) and delayed postovulatory progesterone increase (progesterone at 3 to 5 d postovulation for diets 1 to 5: mean 2.7, 5.9, 4.2, 5.6, 4.3 +/- standard error 0.9 ng/mL, respectively). The number of small (<5 mm) ovarian follicles was positively related to starch intake (r = 0.381) and plasma insulin concentration (r = 0.402). It is concluded that to maintain adequate insulin-to-glucagon ratio in cows at the start of the breeding period, dietary starch concentration should be above 160 g/kg of DM and dietary fat below 44 g/kg of DM, and this should have a positive effect on ovarian function.